At work I always seem to have big problems with algae. This time the first year was fine, but just as it was really starting to roll, I got a real algae/plankton problem. I'm waiting for this to clear up before adding too many pictures.
The algae/plankton is a mix of what appears to be green hair algae and some unidentified nematodes and ciliates. The color is mostly brown, occasionally with some green showing through, and the consistency is like slimy hair. It's not very pleasant. I'll try to get a picture or two from my microscope.
One other symptom of this problem has been a very high pH. A couple months ago I was getting readings of ~8.9 in the afternoon, dropping down to ~8.6 at night. I was never able to completely diagnose this problem, and there was no help from the web. It eventually corrected itself, and now it's about 8.5 during the day. And yes, before you say anything, I'm using a reliable meter, and good water! All the chemical tests I've tried have also agreed more or less.