black worm thing thingy on/under my sand and rocks

[match55]

today just discover something weird, was looking on the sand bed and found some black worm, really thin and small one. After I found that one, I start discover a lot of them around my rocks and sand bed, some even r under the sand.
I am thinking they should be fine in my tank, plus I have no ideahow to get them out away.
Just wanna know what r they and will they hurt if I change to reef tank later on.