Troubles

Let me pose a hypothetical to you all. A player and some others left one FC to make their own. They were feeling pretty run down and generally stuck in their old FC, and not just on content.

Since then, members from the first FC have been making generalised verbal attacks against the entirety of the second FC, while most of the members that left the first FC are offline and therefore cannot defend themselves against such attacks.

These attacks are not quietly done over /tell, but in content runs like T10.

Is it worth the FC master going to the GMs?

Purely hypothetical speaking: YES
I don't know the exact Background of this hypothetical Problem, but from what you told People who are unhappy in their FC left and made a new on, then afterwards the People of the old fc rained slander down on them. If this is all that happened (hypoteticaly of course) then it would be harrasment which the GMs seem to take rather serious.

Short answer: Yes.

Long answer: Yes, and then blacklist.