Scientific Description- Light brownish menace. It looks like snot growing up from the rock or sand, with trapped air bubbles in it. Not to be confused with algae that has an air bubble that has landed on it, dinos make them. Not all species of dinos are bad the one pictured is though, and has caused many aquarists to tear down their tanks.
Manual Removal - Remove the rock and place it in a large saucepan. Add water enough to cover the rock. Boil the tar out of it. Rinse and repeat with scrubbing in between. Let dry for 3 days in sun.
Clean Up Crew- Don't bother.
Why it happened - You didn't quarantine, and you have available nutrients for it.
Starving it out - Use a phosban reactor or a macro like chaeto to take down phosphate. If you have a nitrate problem too, you can add more live rock or rubble to the tank, do some more wcs, add macro, add dsb, etc...
John's Tip - Reduce your phosphates and other nutrients. Iron needs to be controlled. (Everybody forgets about Iron but that can cause problems too.)