Those are nudibranch snails. They eat your zoos.
Zoos are pretty hardy so here is what I would do. It is risky but it will kill the snails most likely. Put the zoo frag (or colony whatever it is) into freshwater. Make sure the water is the same temperature as your tank water. Also dechlorinate the water as well. Then keep the zoos with the snails in the water for like 3 or 4 mins. It should kill all the nudis. I know every time I used to get zoos from someone I would dip them in fresh water and they always survived.
You might have to do this a few times over the next few weeks since the nudis might keep coming back.
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