Our tank is only a few months old and its our 1st saltwater. This is a piece of LR we purchased from our store we got all ours from. This started growing from it a week ago and is growing fast. What is it?
That is an aiptasia anemone and it is a pest that you will want to get rid of as soon as possible. Aiptasia X, Joe's juice, lemon juice, or boiling water have all been known to eradicate them. Just use a syringe and inject it into that sucker.