It does look like a water mite (tick). Has 8 legs, round body, and the size fits the description so I'm pretty sure it is a mite/tick. There are so many different mites and I don't know too many of them, but I do know there are some that are fully aquatic. It may be close to Hydrozetes lacustris, or a similar genus/species that has rounded mouth/head area.
As far as I know, all aquatic mites (not all land mites) are parasites and feed on blood (so I don't think they are consuming algae), so no good news from them :P If it were me, I think I'd sterilize the whole tank they've been in. Now I may be wrong, and these guys may be harmless, but if they are problematic (may attack fish/inverts. Not even sure if some species can travel both on land and live underwater), it may be easier/less of a headache to deal with them now, rather than later (especially since they are multiplying).
Hopefully someone more certain comes along.