Seriously, Mousseline Buttercream (from The Cake Bible) is killing me. I gave it another shot for the Super Bowl cake after some research, and basically the same thing happened, but without the grey, nasty, dishwater-looking liquid.
Here's what I learned... according to a post on the Kitchenaid FAQs, the beater attachment for the Kitchenaid 6qt is burnished aluminum. If it comes in contact with chlorine for a while, it looks kind of tarnished or oxidized. It can pick up chlorine in dishwashing detergent or by sitting in water with a high chlorine content for a while. (Not that I would ever leave anything to soak in the sink while I'm doing something else, of course.)
Barkeeper's Friend (available at Bed, Bath, and Beyond, of all places) and some elbow grease gets the oxidation off.
Armed with such knowledge, I decided to give MBC (Mousseline Buttercream) another shot. Except I sort of chickened out, and just used the wire whip, not the beater, even though the beater was sparkly clean.
As noted, I got the same weird f%@$!@# separation. I still don't think it curdled exactly, it was just very wet. However, this didn't happen until after I had filled between the base cake layers, filled between the football cake layers, and put a crumb coat on the base cake. Last time, it happened almost immediately. So I guess I could call this improvement, in a sad, lame, groping-for-any-light sort of way.
Now that I've ruled out the beater problem, I'm concerned about the temperature of the simple syrup going into the meringue, and maybe the stainless steel Kitchenaid bowl had something to do with it? More research, I guess...
Bottom line: Barkeeper's Friend is a non-chlorinated cleanser that spiffs up lots of metal utensils and surfaces. MBC is darn picky! But I want to give it one more shot (maybe a 1/2 batch next time) because I've heard it's the most stable BC, and the texture (as far as I can tell before it DIES on me) is far far superior to American BC.