You will need to wash it off ASAP especially if you have not gotten any wax or sealant on the car. If it etches into the paint you will need to try a good wash first, it not, then try clay, if not, you may need to use an acidic cleaner which reacts with the fallout or as a worst case it will need machine polishing to remove it.
I always keep a kit bag in my car with clay, MF towels, brushes, spray wax, etc for occasions like that so I can remove fallout like lime, bird poo, etc, immediately (or as soon as I can possibly notice)