Man I've had that exact same think happen a couple of times in the past. It's not the usual RTN that just recedes up leaving a white trail behind it, or just the tissue pealing off. It is a heavy, thick, brownish, fuzzy, snot looking stuff. It actually turns the infected part a different brownish color, before the tissue puffs up real thick, and then starts pealing away. It's almost exactly like the typical brown jelly that usually effects LPS.
Yes it does spread fairly fast and it will spread to any other coral it touches. Mine seemed to originate where two corals were battling it out touching each other, but I'm not sure that it had anything to do with it. It's shown up both on acros and montis before for me. Just appeared out of nowhere. Everything else in my tank seemed to be just fine. It does seem like some sort of weird infection and not really a water quality issue.
Like you I found it better to siphon it off. I was afraid of it spreading elsewhere. I ended up fragging off the infected parts. I found that scrubbing the infected area with a toothbrush, dipping it in Iodine, and then mounting the pieces to plugs, that I was able to stop this thing on several fragged pieces.
I had this stuff going off on a piece of encrusting monti, that was encrusted onto a rock that was definitely NOT coming out of my tank. I ended up doing the siphon, toothbrush scrub trick and then I smeared a wall of crazy along the border edge. It actually worked. In fact it's all grown back now.
What ever it is, it sucks.
