not a nudi. Regardless it's out of the tank now.
I dip all of my zoas using RO water and a iodine-based coral dip or Lugol's. It doesn't take long for nudis to start falling off, zoa spiders can dig in a little and it may take a little longer. Most of the other critters fall off quickly usually in less than a minute or two. Look for sundial snails while you are at; they are cute but deadly - see below. The most important step is to inspect each polyp for nudi egg cases which are spiral-shaped. Pick them off with tweezers. Again I use a magnifying glass or 2X readers to get a close look. It's worth the hassle to pre-treat and inspect thoroughly regardless of where the frag or colony is from. Truth is I pick off nearly everything (algae, sponge, detritus, clams, black stinky crap, etc.) but the zoas, if it's a nasty rock I may peel them off that and remount.