I first noticed it around the bottom of some broadleaf ludwigia. It had little clusters of white balls. Now I was intrigued. The next day, it had moved again into some fontinalis. That's when I hoovered what I could out of the tank.
Myxomycetes are totally harmless things that have characteristics of both animals and fungi.
They eat bacteria on rotting organic matter. Some of the terrestrial ones have brilliant yellow or orange color, and can get quite large. People sometimes panic when they find yellow slime festooning their garden plants, but it's just a slime mold coming out of the compost to sporulate. There are several web pages on these strange and often beautiful organisms.