Could be a Majano. Either way, both are nuisance anemones that you will want to remove from your tank.
If the rock where the anemone is can be removed, use a lighter or cooking torch to burn it off. This works best imo\ime.
If the rock can't be removed, you can inject various things into the anemone with a syringe: kalk paste, hot lemon juice. There are also other commercial products you can use: joe's juice, aiptasia X etc.