i can't tell you anything beyond the fact that it's a dorid nudibranch. it looks very small so likely harmless but unfortunately, outlook on its life is poor. We have no idea what most of the thousands of species of nudibranchs eat. The majority of them are also specialist feeders that often feed on only a single species of bryozoan, sponge, or cnidarian. If your tank runs out of food, the nudi dies. I would not reintroduce it into a coral tank in case one of those corals is the food.
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