Could someone identify this please?

[Delphinus]

You have a couple of options.

You can try a biological control - some people have success with peppermint shrimp (I never have, but don't let that dissuade you). I have a CBB (copperbanded butterfly) who loves them to pieces (pun not so unintentional). A CBB is hit and miss though - they don't always take to feeding and sometimes starve to death. They don't always stay reef-safe. Mine also has a taste for pricier LPS like blastomussa and acanthastrea. So I'd say proceed with caution if you go with a CBB.

You can get a syringe and try injecting it with boiling water, lemon juice, or kalkwasser slurry. Or you can get a commercial product like Joe's Juice or Tailored Aquatic's AiptasiaDestroyer.

It sort of sucks to have to do it but you probably should. In some cases they don't proliferate out of control but if they do, they do so in a big way - you'll end up with 100's of them covering your rock. Not very pretty and they can sting corals so not a good place to be..

Welcome to Canreef BTW.