What are these worms?

[j.marsland12]

For the past week i have noticed these worms in my substrate , sometimes poking their heads out of the surface. I have no idea what they are.
p.s my tank is a shrimp only tank, however i used to have a betta in with them and i used to feed him freeze dried bloodworm but never frozen or live.
Any idea what these guys are? and also are they dangerous to my shrimp?
Many thanks in advance

 

[Gillie]

They look like blackworms they can easily hitchhike in plant roots, they are harmless and are sold as fishfood.

 

[Fdsh5]

+1 looks like black worms to me. My fish go crazy for those. They are harmless and will just wiggle through the substrate.