Catalysis of the reaction: malonyl-CoA + pelargonidin 3-O-(6-caffeoyl-beta-D-glucoside) 5-O-beta-D-glucoside = CoA + 4'''-demalonylsalvianin.
malonyl-CoA:pelargonidin-3-O-(6-caffeoyl-beta-D-glucoside)-5-O-beta-D-glucoside 6\'\'\'-O-malonyltransferase activity