Some things grow in completely on their own without any effort on our part. The Live Rock that came with my tank was pretty much devoid of any obvious life, but after about 1.5 years, I suddenly noticed blue sponge growing on a piece of rock. The white kind is quite common, as well as yellow. The only time I see this blue is when the rock anemone contracts and this patch of rockwork becomes visible. As with any sponge, it must remain submerged at all times. If sponge is exposed to air, depending on the species, it will decay and can foul a tank. When adding LR (live rock) to your tank, be sure to check for any sponge, and if you are unsure you should remove it manually to protect your reef. Other than that, it is reef-safe.