whats this thingy

i got a new peice of liverock yesterday ,:idea2: and out slithered this tiny slug looking thingy about 1/2 " long looks like a maggot with tenticles , but when it let loose the rock and sorta floated across the tank it opened up to a little flat oval disc ???? very pale yellow with white lumpy spots on it , no shell at all,

also just spotted a grayish /black spidery thing with threadlike legs that launched up off the rock then formed a ball and torpedoed into a hole ,it was so small it could sit on your pinky fingernail and i am amazed i spotted it at all with these eyes,

hope you dont mind a thousand questions comming I am brand new to the salt tank set up here , my hubby and i did it all backwards thanks to a lfs , we were going to go straight fish now slowly converting to reef, and reading a thousand books , any advice would help, :bowing:

thanks , ill have to watch it if i can find it again , and the crab thing is to tiny to see on camara thanks for the link