I would hate to say this, but that looks like aiptasia. It's like the weed of this hobby. It's a stinging anemone that is very difficult to kill, and has been bothersome for us fishkeepers for years and years.
If they don't get erradicated soon, they will multiply and take over your tank. Get some Joe's Juice and get rid of it before it spreads.
I'm so sorry to be the bearer of bad news! You seemed so excited to have it in your tank!