I wish I knew what caused this.... Mine has a similar weird blob on it, still healthy. Don't worry about it, they do fine with the weird blobs. Here is a picture of mine 8-9 months ago and it's still doing great.
<a href="http://s1339.photobucket.com/user/mrturritos/media/id_zps91ccf061.jpg.html" target="_blank"><img src="http://i1339.photobucket.com/albums/o702/mrturritos/id_zps91ccf061.jpg" border="0" alt=" photo id_zps91ccf061.jpg"/></a>
The only thing I wonder if it is caused by aptasia stinging the anemone, I got mine from a tank over run with the pests and it's the only thing I could think of. Everything I've read says it's just some random thing.