I put gumpaste funky lettering (using tappits) on a buttercream cake. Put it in an airtight container last night and when I woke up this morning, the letters looked wet. Seems the gumpaste is absorbing moisture. I took the lid off so it could "breathe", but this cake is for tomorrow and I don't know if I should leave it uncovered for that long. If I put the lid back on, will the gumpaste melt, or the coloring bleed?

I leave cakes uncovered after I've frosted them all the time overnight. The frosting keeps the cake from getting air to it and keeps it moist. I would leave it uncovered so the gumpaste can dry.

I never cover my cakes.

is it crusting or regular bc?

Don't cover your cake. I keep mine in the fridge because I use real butter...but they are uncovered and any gum paste or decor holds up pretty well. Unless the letters are melted, just leave them alone and they will dry.