That looks like aiptasia, or a related anemone. They are generally considered a pest hitchhiker, as they can sting corals and spread rapidly. There are different ways to get rid of them. Peppermint shrimp allegedly will eat them although I've never seen it. You can spot feed them a product like Joe's Juice or Aiptasia Rx and that will kill them if the tentacles are out and they take some of it. I just did that with one in my nano 2 days ago, and no trace of it today.