Is my sponge sick

[cheryl]

We recently added a sherbert sponge to our tank. Our hermit crabs and pepperment shrimp have spent alot of time crawling over it and appear to be eating algae off of the sponge. My concern is that sponge has developed a two toned appearance and I am wondering if it is sick or dying or if there is something that we should be doing to help it or is it just a happy normal sponge. Anyway here is its picture any help would be greatly appreciated

Thanks