Saw this on my damsel this morning… impossible to catch the fish so far . Can anyone tell me what type of pest it is and if it’s going to come off on its own?
That looks to me like a parasitic isopod of some kind - from the few I've read about, if yours is still growing then it may jump around from one fish to another occasionally. Some species want/need really specific host species (at least as adults) and typically settle in specific places on the preferred host fish once they find one (probably the most famous example of this is the species that settles on its host's tongue).
Either way, these parasites are known for causing serious harm to their hosts (they've actually been known to kill their hosts and move on), so I'd recommend trapping that fish and removing the isopod if at all possible. Reefs.com recommends removing them using pliers:
I have been battling Cirolanid isopods for a few weeks. Have caught 35+ thus far. Luckily a fishless system still, so I have time on my side (although it feels never ending). Best approach I have found is very manual, but requires a small net, flashlight and patience. After dark, periodically, look around the water column for anything sputtering around the water and try to check over your rock work and sand for them. Manually scoop them out and discard (I place into a bowl of freshwater until they are dead).
Hard to tell if that is also a Cirolanid, but best way to tell is their eyes. Here are the first two small ones I captured.