The good news is that mostly looks like a magical shimmer, especially if Kuja is also keeping his tail close to his body, so it's less obvious that the magic is attached to a tail rather than his torso[1]. She probably won't say anything unless she needs to know, needs an area clear of others' magic, or there's a reason to suggest they aren't just cosmetic or protective.

(It does mean that she might have enough of a sense of Kuja's magic to start to recognize it, but she's also seen him use it anyway.)

[1] I mean she helped revive him, so she probably knows about the tail anyway.