tang fin mass

Hi everyone, I bought this yellow tang yesterday from my lfs, I have noticed a slightly discoloured patch on his bottom fin. He is eating good already, picking at the rocks, eating pellets and also frozen foods. He doesn't seem to scratch at the rocks and seems happy and swimming up to now. My...

Can anyone please diagnose this mass on the dorsal fin of my mimic tang? It appeared overnight and has slowly grown in size over the past 2 weeks. Almost looks like I can just pick it off but I can't catch the fish. Could this be some sort of parasite and will it fall off on its own?