What is this stuff? How bad is this stuff... what to do?

[Heidi]

So I pulled out a few chunks of zoas today cause some of them were not looking so "hot". Granted I just came off of a 2 day lights out which I am sure is part of the issues. But I have these fantastic green zoas that have been trying to "melt" for a while now and I am BOUND and DETERMINED to not let that happen. SO I have been moving them around to get them in just the right spot, and today I decided to give them a little dip in the old Coral RX/ iodine cause I had some melting zoas a month ago that I saved and are starting to look great now by this combination. So I grabbed them and a couple other "not so great looking zoa. The ones in the picture were really not looking too bad and open mostly. But there are others in the tank that have these whitish blotches and are not faring as well.
So what is it? Is it killing my zoas? Can I get rid of it somehow? How?
I can scratch it off a bit with my fingernail.... not all the way... and the coral rx/ iodine didn't seem to do anything to it...


any help would be great guys!
Heidi