If you can catch it, get it out and flush it. I had similar nudi's that were eating my SPS. Don't put it in your sump. It could still end up in your tank. Without a positive ID, it's not worth the risk.
this is an upclose picture, this creature is only about a quarter of an inch long, pretty cool looking, I'm not going to put it in my sump, just in case, i don't need it laying eggs and taking over my corals
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It's a nudibranch and they have specific diets. If it has a taste for anything in your tank, it will eat it very quickly. They have voracious appetites for their size. I get paranoid when introducing mussel clusters into my Coldwater tanks. I pick through them thoroughly. Especially after losing a prize colony of corynactis to one.