Just noticed this a couple days ago on my gin ren.
I put a little Mela fix in there this morning but not enough to complete the dosage only had a little bit in the bottle mother bottle. Getting ready to do a partial water change and possibly locate him to a quarantine tank. It was hard to get a good picture of it but it seems to be developed around the fin and he has a spot on the side of his body near the tail. A few weeks ago I added my UV light, partial water change and I killed a pretty good algae bloom and normally do a partial water change afterwards but i’ve been busy not been able to get to it. The water is cleared up now but I have not checked my water parameters yet. Will do that tonight.