i was thinking it could of bean a lack of molybdenum in the water which can cause corraline to loose its colour and turn white allmost over night.
there is an calcareous algae that is white but i cant remember the name of it at the moment, but to get it you would need good lights and high levels of iron and calcium for it to grow.
so what you are discribeing is a hard white covering on a peice of rock. if this is the case i am at a loss as you dont drip kalk
or use addatives.
i dont think it would be a sponge but it dose sound like a calcareous groth of some kind. TBH just keep an eye on the water conditions and if all seem fine i wouldnt worry at the moment. if it starts to spread or water conditions get worse then i would take some kind of action, but im not sure what at the mo..