This nudibranch is a member of the Discodorididae family.  It looks so much like the red sponge it feeds upon that, unless it moves, it is extremely difficult to notice.  Its body is covered in blotches of white and cream (and often sand)  with a central crater that makes it appear even more sponge-like  The individual here was found in about 6 feet of water at the edge of a broken and exposed rocky reef.  This one was about 3 inches (75 mm), but they can apparently be as large as 4 inches or 100 mm.  For more info see Sea Slugs of Hawaii.