Almost lost my cowfish...

[Coralgurl]

I don't know anything about these fish so take this as you will. What about a dip in something medicated? Weird that he'd be growing algae on him but its finding nutrients there likely in the dead flesh. I wouldn't want to pick it off as I would think you'd risk doing more damage.

You should post a pic so others can help with diagnosing and ensuring you are getting proper advice.