anyone know what these are?

[mikeoz]

Found on the back (dark side) of a large slab of my live rock

 

[Sk8r]

Could be the start of sponges. Sponges in various colors tend to be, well, spongy, sometimes domed, sometimes irregular, and usually prefer the dark. They're helpful filter feeders mostly. Very few will ever give you trouble and most are nice additions to a tank. Just keep an eye on it, if that's what it is.