I had a green fuzzy mushroom on a snail shell for almost 3 months in my 20L, and suddenly it just decided to release. I guess some things you just can't figure out in this hobby. Since then I've been treating it like a baby and finally got it to reattach. Now if it was anything else I'd be worried, but shrooms are resistant to a lot.
And FYI, great tank. Do you dose for anything?