A macro shot I think would be the determining factor. The opaque whitish coloration lends itself to that of a sponge, but I have never seen sponge trail in straight line so to speak as in the picture above. Using a dedicated set of tweezers, if you can grasp any of it gently, and it pulls away, it's sponge. If it's flat and smoothly attached to the rock, I say it's the Stolen ( or runner which connect the polyps at the base ). Just my 2 cents.
Mucho Reef