Google drain fly larvae and look at the pictures. I had some nasty looking things once in an hob filter that were these. I wasn't cleaning the gunk out good enough and since it wasn't a closed system like a canister, they got in it. I don't know if they can get into a canister or not but I haven't seen them so far. What kind of filter do you have?
Here is a link to an old post of mine that has a couple of pictures of what I'm talking about.
It is most likely a bloodworm, they eat detruis and wood I get colonies of them in my pond and any where water has sat for periods of time with wood or leaves in it. Bloodworms are just the larval stage of gnats and harmles to you and your fish, and fish love eating them so a handy snack to keep a colony of.