it sounds like a stoney type algae to me and if I'm right that's not a bad thing. Is it flat against the glass? If it's coraline algae or stony algae a razor blade can be used to scrape it off the glass. (don't know how you get if off acrylic.. the green and purple types are all over my tank- I don't bother with it except on the front of my glass so I can still see.