I have a maroon clown that gets small spots like this at times,but not as large as yours. I think mine does it by rubbing against rocks and sand because it is always stirring up sand,darting into the rocks, and chasing it's reflection.It tatters it's fins too but it always gets better.I tried to calm it down with an anemone but it buffed it too much and killed it. They seem to be hard for me to keep.I know it may sound strange but I have green ricordea and my clown does host in it because it does look like an anemone but it is easier to keep. Keep an eye on yours otherwise till you figure it out.