Tiny lil flatworms on my glass, about 1mm long, two tails on them. Not sure the exact species. Don't seem to bother anything and they seem to like the glass and flow.
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Benign, unless their populations get out of control. They can cover up soft corals in extreme cases, use a turkey baster to launch them into the water column, such that fish will eat them. Alternatively, consider adding a wrasse to your aquarium, assuming this fish is compatible with your reef. I have a few in my aquarium, always in very small numbers, for years, never worried about them at all.

