Chocolate chip star

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Hi all,

I'm having a hard time finding any info on this spot that I just noticed on my star. I have a 60 gallon FOWLR that has been going for about 2 years and this guy has been in the tank about 3 weeks. He's pretty active but stays on the glass, mostly at the top. I feed him a chunk of krill every 3ish days and it takes about a full 24 hours if not longer for him to finish. Anyway, I noticed this spot on him the other day and I'm not sure if I should be concerned.


It's not moving but it looks like it has flattened out a bit today... The top pic is today. My salinity is at 1.022, amonia 0, nitrite 0, nitrate 10 & pH 7.8... if it were a disease, wouldn't it likely be white?