Hello,
I do not think the Sponge is EATING the SPS.
the SPS is unhealthy and dying from base, and sponge growing on its place. [looks like a packed SPS which this happens to, like the birdsnest look alikes]
what makes a sponge grow that fast ? too much bacteria in water, or too much nutritions for it [other forms]
the tip color of the SPS [which is supposed to be green] confirms this too.
run filter socks, do more water change, skim wetter. basically bost the health of your SPS.
for this piece, cut off a couple of tips, and frag it, and leave the rest as a sponge colony
Ive had this happen to my birds net before, and my ora green stylophora.
HTH,