I've been observing this growth in my 40 gallon for about a month now. Initially, I thought it was a Hydra, but it just keeps getting bigger? Doesn't appear to bother my Harlequin Rasboras at all. Tank was planted in Mid February of this year. Here are a picture of the strange growth on my filter, and what I believe to be it's relatives around the rest of the tank.
If you look closely at the picture of the filter, you should be able to see smaller similar growths as well.
What is this thing?