[Tangs Rule]

I have a 150 gallon FOWLR, and I noticed that on my orange shoulder tang, my emperor angel and my lutescens wrasse that each of them had brown on their skin (not exactly spots but more like patches and it's a light brown.) I noticed the wrasses "scratching" itself against the sand. Does anyone know what this is or what I can treat the tank with to get rid of it?

I have a wet dry fiter with UV sterilizer and protein skimmer on the tank as my filtration.

Thanks