I've been growing algae on rocks for a few weeks for my gobies... the shrimp seem to like it too. I have found though, that the first rock I did was great. lush and green film of algae on it. But since then, I haven't gotten the same results. A film forms, but it is more light brownish. I'm doing this in a rather small container, so I'm not surprised, but the question remains, how is the best way to grow desirable algae without triggering hairy junk?