that is a type of macroalgae growing in it. I do a peroxide dip to rid most of those. I just add enough to the water to make it fizz good. Ive read people doing 50/50 peroxide/water. Might have to do multiple dips. Some macros are more resistant than others.
yeah I come across one every once in a while that does the same. I keep a QT running all the time and when I get something real stubborn I just place in there for a while starves everything out it seems.