Those are stretched bristleworms. They appear to have sparser spines when they stretch a bit. They're good, but if you overfeed they can multiply out of control.
Definitely don't touch, they'll get bristles in your finger and that will hurt. Unless you're allergic, they aren't dangerous. I think it's bees and ants that can mean you might be allergic to bristleworms...
You can't have too many, not really, but they can be an eyesore when you have a few thousand. If you don't mind the look, they're fine. If you don't want too many, you could try a pistol shrimp... Mine eats them.