New driftwood, I assume? If so, it is a pretty normal thing to find that will soon go away on it's own. You can reduce it at times by snagging it out or just wait a bit. Nothing that takes over the tank but just some little mystery organism who has found just the conditions he needs for a time. Likely to go in a couple weeks as the conditions change. Seems no harm to fish as some will eat it.
I do agree that it is ugly!!