FloridaFishGuy

If you decide to go to the extreme, a tank treatment of Erythromycin should take care of it in about a week. I dosed it at half what I would dose if I was treating a fish. Be sure to take carbon out of your filters.

I had something similar to this pop up on a piece of spider wood I bought on alibaba. I did some research and the best thing I could come up with was Xylaria sp. That may not even actually be what it is, but it’s the best I could find. It popped up in a 5gallon tank with a Betta who ended up passing randomly. I did some further research and found it is slightly toxic, perhaps explaining the death of the fish