That's a green implosion paly. Unfortunately they can be real aggressive, tend to sprout up randomly around the tank, and can sting other softies.
They'll eat xenia. They also gorge of meaty food that your fish miss and this will rapidly speed up their growth. That won't grow as fast in a low nutrient tank.
Attached pic of how fast one can grow in six months in one of my 10 softy tanks. This one was liberally fed a lot of protein. The gobstoppers are big enough and grow dense enough to crowd them out.
_MG_2310 by
blasterman789, on Flickr