I have to agree with Jesse 100%
I have two of the same, that have been in my reef for about three years now. They are harmless and actually grow full size to abut the size of a golf ball, so I have been told. I had Dr. Ron send me the true latin name for these guy's but I just can seem to find the email
As with most sponges, they need to stay submerged all the time to stay alive, so if you let them contact with air they won't be around soon after.