If they aren't conformal coated it is going to be a problem as there are lots of spots for the residue to hide in.
It used to be rare for rocket electronics to be conformal coated but the last one I bought (ALT15KWD) was, so that might be changing. If you are lucky enough to have a can of the stuff you can do it yourself. I found one at Fry's Electronics. Of course it doesn't help now.
The residue is most likely water soluble but the catch is that the water will combine with it to make acid. Not a good idea. Try isopropal or denatured alcohol. Do not use rubbing alcohol as it has oil of wintergreen added. Be careful about pressure sensors. Pressure sensors have their own protective coatings inside the case so tape over the port to protect it while cleaning.
I had this problem with a GWiz LC Deluxe a while back and it cleaned up pretty well although I had applied a conformal coating before flight.