Aquariums: What are these?? I don’t see them in my main tank, but found them after siphoning out some of the substrate

There must be thousands of these little things… everyone in my tank seems to be doing fine expect one rasbora and a failed shrimp molt, so I’m hoping not related. My nitrates got high on accident leading the the shrimp issue I think. Is this is infestation? Are they harmless??