Thusly Spake Unto All:

reappeared within two weeks. We would like to know 1. what they're
called, 2. if they are beneficial or harmful, 3. how they are spread,
4. how to treat if necessary.

They are likely nematodes, catenulids or aquatic earthworms
("tubifex") feeding off leftovers and turtle-doo. They are highly
unlikely to be parasitic.