I'd like throw in a little personal experience...
Two weeks ago I realized that my dKH had drifted up from around 9.0 to about 12.0. During this same period, my tank's avg. and max daily temperature had drifted up about 1.5 degrees (my apt. complex hadn't switched over the A/C from heat).
For over a year I had a Bali aquacultured A. mille in my main tank; 3-4 weeks ago, I moved it to my frag tank. The coral's growth had always been extremely slow and had long since lost its vivid color (BRIGHT green -> yellow/brown). It was no longer a "showpiece" but I still didn't want to throw it away, so it went to the frag tank. Anyways, when the dKH and temp creeped up, both pieces (I split it in two to experiment with lighting needs; one was on a high shelf, the other was down low) got RTN. The lower piece (also in an area of much lower flow) got it very bad and was removed from the tank; the higher piece didn't look TOO bad (it was more like STN), so I decided to keep an eye on it. On the same day I noticed another Bali aquacultured piece in my main display had started to get STN.
Within 4-5 days I decided to act upon the coral in the main tank b/c I spotted what appeared to be AEFW eggs! I removed it from the tank and blasted it with a turkey baste. Sure enough, I had 3-4 flatworms pop off (2 types actually - one was completely clear, the other looked more brown like the ones Bigred and weatherson captured here:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=756327).
Last night I went Round 2 with chilled SW + Lugol's. 1-2 more small FWs came off the coral that was in the main display, and 9-10 came off the last piece of A. mille. I didn't find any eggs on either piece, but the A. mille was removed and put into a test bowl with the AEFWs for observation.
FWIW, I haven't had any Acro crabs for almost a year, but I do have a Sixline Wrasse in the main display. My only thoughts are these: the fish kept the FWs at bay, but once the A. mille was moved to the frag tank, they were able to populate. Then, the elevated dKH and temp may have stressed the corals, making them easy targets for the AEFWs. Although, it could all just be a coincidence; maybe I inadvertently added the AEFWs here recently after a local trade or frag swap - after which I did QT, but not for 3 months like One Eye has suggested.
HOPEFULLY I've spotted this early enough to do some damage to the AEFW's life cycle. If this is now isolated to the frag tank, do the FWs have the ability to migrate to the main display? I would think that the bodies or eggs would get destroyed by the return pumps (Poseidon T1s).