Hello,
I have worked with quite a few sponges and sea squirts. Its difficult to determine exactly what you have from the image (focus). However, it does appear that there is a large aperature (sea squirt) or osculum (sponge) in the lower or side of the organism. There also might be one on the top, but it is difficult to tell. The typical configuration for a sea squirt is to have a branchial aperature (siphon) on the top and a atrial aperature (siphon) on the side or lower area. The way to definitly determine what you have is to lightly poke it with a piece of rigid airline tubing. If the organism contracts, it is a sea squirt. A solitary bright pink sea squirt is a nice animal to have in your reef. Prefers to consume phytoplankton and fine particulates.