so, in my sump, in the chaeto, are a couple of these. this one is about 2" long in the pics. the other is much smaller, maybe 3/4" and thin like a small pipe cleaner. both smooth exterior. clear skin showing visible digestive tracts that straighten when the worm extends to full length. they retract from bright light, they seem to be filter feeders.
i cant find them online except one shot on eXtreme Aquatics that isn't identified but might be this. if it isnt a pest ill gladly move the topic to inverts. lol