I believe peanut worms are harmless. I have a few in my tank and have not noticed them causing any trouble at all. They seem to spend most of their time in holes in the rocks. I don't know for sure but I believe they actually burrow these holes themselves. That's good stuff, it increases the surface area of the rock allowing for more denitrifiying bacteria. The peanut worm in that pic is fully (compressed?) is that the right word? They do stretch out pretty long when moving.
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