What is this?!?!?!

[Tahoe61]

Would need macro shots to see the nudibranch eggs. This is what they look like (not my image)

 

[Lionfish Lair]

Get a net nearby. They are really hard to grab with your hand and remove from the water.

 

[Preme]

Yes remove that asap!

 

[redfishbluefish]

Those zoas appear to be attached to the rock. I'd be very reluctant dipping those in any coral dip.

If dipping the entire rock was last resort, I'd then put that rock in it's own small QT tank and do total water changes every day to make sure all the dip leached back out of the rock.

 

[Ranjib]

I would echo same concern. It's likely a spa eating nudi. Use Twizzer to remove any of the adult one you can spot. Do a dip, to cleanse the rock. Be thorough on making sure dip does not leach on display tank. Either keep the rock separate on qt, or do multiple rinse with fresh saltwater, after dipping.
It is likely you have bought a coral that is harvested from ocean floor, not aqua cultured. This is a common problem with such corals. I have learned this hard way. It's safer to get aquaculture corals in frag plug, even if they are small colony (couple of polyps).
If you do end up seeing more nudis (check zoas heads at night. They'll be there. Or check if zoas spas are stayibgn closed)., use flatworm exit. Or wrasse. It's easier to control them when they are detected early.

 

[Vahanyos]

That's a big nudibranch!

 

[David Reiche]

We found one more, but none since 2/21/2017.