What are these

[hillegom]

I think we all have these worms. Can be considered part of your cleanup crew.
As mentioned, their numbers will wax/wane depending on the amount of food that is available to them.
Can be burdensome if they get too large, as in 4-6 inches. But some people state they don't mind them large.
If a problem, place some shrimp, leftover food into a piece of nylon stocking overnight. The worms will catch their feet into the nylon, and in the morning, chuck the nylon.

[ponokareefer]

I've got a monster one in my tank. I haven't seen both ends at one time ever. I haven't seen any ill effects from it being in my system. I do see it eating excess food if there is any.