I had the same thing come up a few days ago. The larger consensus was detritus worm, which means poop eater. Not bad, just gross. I'm still on stand by waiting for any input as to whether they could have a negative impact on the tank. So far I've gathered that the population is dependent on the bio-output and will die off rather than overload the tank. Not sure if that's right though. Hopefully we both get some answers.

 

Nematode! At least that is my guess based on the pic. Its 100% harmless. If you doing like them just feed less and clean your tank better. I keep a lot of nano fish and that there is a tasty snack to them . My canister is home to a colony of planaria, adults get almost an 1" long. Population of these things is dependent of excess waste and food. Before I started finding planaria I had a lot of cherry shrimp in that tank. Two angel fish removed all the shrimp from the tank. The waste the shrimp had been eating was now unclaimed and poof suddenly there were planaria.

 

Yea they're harmless. I've seen my ghost shrimp eat them.
I had some little dots swimming around at the same time, but the shrimp ate those too I think.