No, it's probably OK, but you could get it checked at a decent radiator shop. Even if you don't pay them to diagnose the problem, they probably won't charge you to check a cap.

While you're at it, check the hose from the filler neck to the overflow tank and the seal at the lower hose. Drips at these locations wouldn't leave much residue, so look closely. Also, look ALL OVER the block for areas that are cleaner than their surroundings (steam cleaned) and see if anything is coming out of the bellhousing. Freeze plugs and head gaskets can have intermittent leaks.

Steve
83 Custom w/95 4.9L EFI, 78 NP435, 83 NP208, 83 D44IFS 3.07, 87 Ford 8.8ABS 3.08, Michelin LTX AT 31x10.50R15