I made a birthday cake yesterday, i frosted it in american buttercream. 1 stick of butter and 1 1/2 cups of shortening.  The cake was kept refrigerated because of the filling and then kept in an ac room for the party. I took the leftover cake home, refrigerated it, covered with plastic wrap, and this morning the frosting has small white dots all over the cake. What happened? How i can i prevent this? Never had this happen before. 

I have no idea!  If it were me, I would taste of the white dots to see if I could determine if they were butter or powdered sugar.  Knowing what it is might help to solve the problem.

It’s a big guess, but could be small hard pieces of icing sugar. This happened to me once when I was in a hurry, and didn’t sift my icing sugar. Little tiny hard pellets of icing sugar popped up to the surface. But mine appeared within a couple of hours. Strange it happened after 24 hours. Of course, it’s an educated guess on my part.

Boy what a puzzling thing.  I’m wondering if it could be shortening that didn’t get mixed up.  Did you beat the butter & shortening together really well? Like for 5 eve  up to 10 minutes before adding sugar & as June said soft it 

Just a thought...could the spots be from condensation, the beads of water maybe left a wet spot.  I'm just thinking sometimes I have things covered with plastic wrap in the fridge and there is condensation.

As I said before  I thinking along the lines of it being from not creaming the butter & shortening well enough like somewhere around 8-10 minutes.  This might be especially so if it was not a quality brand.  It also could be any of the other possibility offer