I had an 800ST and on a very few occasions it too had that white stuff, not so much of it, drip from the radiator, or the cap, or somewhere.
It has been reported by a number of 800 owners. I don't know if anyone ever found the cause and the cure. If they did it may have been reported on bmwf800ridersforum.
I just got back from my own ride down to Cabo on the 12th.
It's easy to cross into Mexico at Tijuana. Nobody gets the visa or pass, so I walked in and got it in 5 minutes. But coming back at Tijuana would not be a good idea. That's where Tecate comes in nice.
Or, Mexicali, where motorcycles are allowed to cut to the front of the line, thru' the Sentri lane.