Bad. Thats an aiptasia.
Don't fret tho, stay in this hobby long enough and it will eventually end up in your tank.
Many ways to effectively manage it and keep it under control. Compete eradication is a fruitless exercise bc eventually it will come back.
Just look up aiptasia treatments in this forum, many different ways to treat from how to deal with only seeing 1 or 2, to how to deal with plague like proportions