Petrolishthes caribensis - Porcelain Crab Their large claws appear dangerous, but they are peaceful and plentiful hitchhikers. Very Common Hitchhiker   

Pilumnus sayi - Gorilla Crab Hairy crabs make great puffer and mantis snacks! Seasonally common hitchhiker   

Pincushion Urchins are pretty and prickly!  

Orange Ball Sponge looks like a cheese ball, but don't eat it.

Bryozoa: Looks like a fine brown macroalgae, but they are actually a colony of invertebrate filter feeders.  Bryozoa are common on live rock in the cool water winter months.  

Delicate looking stalks with miniature pale filters attached to the live rock could be a type of hydroid. Some say they sting corals, others say they spread like a rash, but connoisseurs find them unique. Hydroids are filter feeders, so phyto, oyster feast, will keep them looking good. Or, you may choose to remove them.