One of my tanks has developed a blue green crud in it. It's not algea, at least none that I am familiar with. I covers the rocks and such in a layer. I vacumn it out but it's back seemingly within hours. Does anyone know what this is and how I get rid of it? My Pleco won't touch it. It's not causing a problem excpet for appearances, but I don't want it in my tank.. TIA! Ty