[reefnoob1991]

Hey guys, 3rd day of having my tank and I came in and on my base rock theres this greyish white fog looking stuff but its stuck to the rock. I can't get pics cause its way in the back of the tank but when i stuck it with another shell it kinda floated apart in pieces. What is this?

[Reticentregret]

have any coral yet? if so look up coral spawning .. or coral smoking.. their sperm looks like a gray smoky cloud tends to get caught up on things very uncommon though specially in captivty

[reefnoob1991]

I just got my tank so there's nothing in there, and its attached to my base rock, which is very weird

[ffd649]

I notice the same thing in my tank and found this discription with an identifiable pic on Saltwaterfish.com Message Boards - Hitchhiker Identifications

It has an entire listing of hitchhickers that can be identified, a big help.

They called it a sponge
Sponges: (white spiderweb looking stuff)

To date, there are over 5000 species of sponges described so identifying a particular sponge can be quite tricky. Most of the hitchhiker sponges I have seen are filter feeders, feeding on tiny particles, bacteria and/or dissolved organic matter. Photosynthetic sponges often die off during the transportation of the live rock but that is not to say some will not survive.

ALSO Q-tip sponge (looks like top of Q-tip)
Q-tip sponges: As the name implies, these are sponges that are about the same size and shape as the end of a q-tip swap. They have a small opening at the top, and can also look a bit like a pineapple. Like spirorbid worms, they are often found in young tanks. They are harmless filter feeders that also tend to decline in numbers once tanks mature.

hope this helps