A while ago I said that I'll stay away from Mollisia but I guess I have to take my words back. 
I was reading Fungi of Temperate Europe and the title's species struck me as quite distinctive and something that might be possible to identify. (Belonopsis retincola is apparently a synonym and Species Fungorum claims that Trichobelonium kneiffii is the current name.)

So I visited a small pond nearby that has Phragmites australis growing in it. I found these almost immediately, growing on dead, standing culms near the waterline.

This time I managed to get a clear yellow KOH reaction. The medulla has plentiful crystals in it and the spores measure 23.4 - 26 × 2.8 - 3.4 µm. 

With the combination of these characteristics, I'm quite confident this is indeed M. retincola.