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If the white mage were the only one that could cast that buff, then it wouldn't matter, but two classes can cast an inferior version, and sometimes the whm can't keep track of everything everyone is doing.
You can see if a buff is yours or not by the different color duration indicator. If you're worried about a WHM not knowing whether it's a Graniteskin on the target or not, unless there's another WHM in the party (wherein the only other possible user would be the PLD, who you'd be able to tell if they're casting it by the fact that they're *casting*), then they'll be able to tell just off of that.