I believe it is Aiptasia that you have in your tank. You should get rid of it as soon as possible as they tend to spread rather quickly and can become a nuisance to say the least. DO NOT try to scrape it off or break it in to pieces as they will regrow individually. They are dangerous to your livestock because they are extremely aggressive and have a strong sting.
They are however a common intruder and are relatively easy to deal with. Using a syringe you can apply Aiptasia-X or Kalkwasser until it recedes and dies off. You can also use Peppermint Shrimp that have been known to eat Aiptasia. However, they may be a hit or miss.