[Van down by the river]

Some sort of worm. Not even going to try to guess what it is.

http://www.mywhitecourt.com/gallery/...08&id=worm0021This worm is a peanut worm.
Sipunculida sp.
A harmless detritivore living in your rocks.

http://www.mywhitecourt.com/gallery/...8&id=ident0005The green algae growing on your dead coral is Green Boring AlgaeOstreobium sp. If problematic in your tank it can cause tissue recession in affected corals.

http://www.mywhitecourt.com/gallery/...=ident0003_001
Another Green Thingy is as Christyf5 said Acetabularia sp.
Possibly Acetabularia dentata sp. or Acetabularia polyphysoides/Polyphsa polyphysoides
Most in this genus are small and very decorative and unlikely to become problematic. I've had them a few times and thought they were quite attractive, unfortunately my tangs found them tasty.