Myath could benefit from Coincounter's treatment

[Deslyxic]

The change to telegraphs of the Coincounter is already a headscratcher, seeing that SE decided to change after months of people farming Aurum Vale for them moogle TS.
But as I reading that, I got Sohm Al on a leveling roulette, and the tyranosaur boss (Myath iirc) could really do with proper aoe and stack markers on players for the slime throwing attack. Just saying

[Rongway]

Not every stack mechanic needs to be marked with an explicit stack mechanic marker when the mechanic is actually "discern between these two mechanics".

The Myath fight in particular features two opposite mechanics that you're expected to figure out based on color of the slimes. Figuring out the difference between red and blue is enough. And it even meets accessibility guidelines by using additional non-color indicators! The mobs are named Blood vs Rheum, and the marker shapes are distinct from each other.

Yes, we definitely have an interest in consistency, so fights that feature stack mechanics as their own mechanics should have stacks marked. But when two mechanics are happening simultaneously or in rapid alternation, I think it's fine not to mark the stack part as an explicit stack and let the players discern the two mechanical components from each other. For comparison, it would be little different from pairing or alternating a PBAoE with a donut, and not explicitly marking them for the majority (or entirety) of the castbar--for example during Qitiang Dasheng's split, when they simultaneously use Both Ends, you're expected to identify the one with an oversized staff as the donut and run to him.

The game in general seems to agree with you, however, and has been pretty generous even with these types of mechanics--for example, Alexander Prime's orange and green, or Omega M/F's orange and pink; both explicitly mark the stack part of their respective mechanical complexes.

[Genz]

Basic design 101 : If it has the same function, it must have the same appearance.

So yes, stack marker for Myath.
Also stack marker for the Lightning attack from the Hydra in World of Darkness (currently a purple marker)

[Rongway]

Basic design 101 : If it has the same function, it must have the same appearance.

This is definitely a UI/UX design rule, but not necessarily a video game encounter design rule.

[Deslyxic]

Not every stack mechanic needs to be marked with an explicit stack mechanic marker when the mechanic is actually "discern between these two mechanics".

The problem is, for starters, that is really hard to decipher unless you're told how it works. Sure they mechanics have different markers, now what do I do with the different markers? I don't know.
Also even when you know the mechanic you can get the two attack mixed very easily, I know I did, I got it inverted in the last run and had no clue until the healer soaked alone a red slime and got killed.

[Frizze]

If i remember right, the multi-headed dragon from WoD uses the same marker that Myath uses for the stack slime(whichever color that is). It was later in that same expansion that i believe they switched over to the current stack marker. For consistency sake, i wouldnt be against them going back to any of these older instances and changing the marker to match the current one. People can start(hopefully) learning what to do with stack markers earlier on and be more ready when something like an Ahk Morn is tossed their way in Final Steps.

[ItMe]

I wonder if this means they'll soon add AoE markers to the cyclops monsters in PotD (since they use the same moves).

[Espritduo]

I've run Sohm Al dozens, if not a hundred times over the years and to this day I still never remember which is which in that fight because it is absolutely non-intuitive. Of course the main reason I never bother to remember is because it is also pretty inconsequential and never appears again anywhere in the game. The damage is small enough that even if you screw it up it shouldn't matter. I've taken a stacked marker by myself plenty of times as a DPS and maybe taken half my HP in damage, and eaten splash damage from the spread marker and not even felt it.

[Rongway]

If i remember right, the multi-headed dragon from WoD uses the same marker that Myath uses for the stack slime(whichever color that is). It was later in that same expansion that i believe they switched over to the current stack marker. For consistency sake, i wouldnt be against them going back to any of these older instances and changing the marker to match the current one. People can start(hopefully) learning what to do with stack markers earlier on and be more ready when something like an Ahk Morn is tossed their way in Final Steps.

Five-Headed Dragon uses a purple marker for the lightning stack. The lightning stack should probably use the universal stack marker, as it's not explicitly paired with an opposite mechanic.

I wonder if this means they'll soon add AoE markers to the cyclops monsters in PotD (since they use the same moves).

Coincounter is conspicuous in that it's the first Cyclops that most people encounter. Since players haven't had any exposure to cyclopes by then, there may be a design interest in telegraphing the attacks for that encounter so players can see the difference between a Swing and a Swipe. In later encounters it's fine to expect people to recognize that a mob is in the cyclops/minotaur family and dodge based on previous knowledge of cyclops attacks.