ID Help on this, please!
 

Found under rock with a large zoa and shroom colony. Didnt notice it until I put it in the aquarium. I did a 10 minute dip in revive.

It was hanging out of the rock and then sunk back in after.

https://imgur.com/a/oOZdH

Also there is some type of nudibranch that came off as well.

Thanks!

That's probably a cucumber.

Appears to be one of the little black/brown cucumbers that are prevalent in Indo-Pacific rock. They're filter-feeders, similar to the common yellow colochirus cucumbers. Totally harmless, won't damage your tank even if they die, and while they live they constantly process organics out of your system.

As for that second pic, I'd vote flatworm rather than nudibranch... some of the giant flatworms can contract their edges into some wild shapes when they're exposed or threatened...