Replace the cap for now...it's around $10-20 depending on where you buy it. Mine did that once, but it was due to not screwing it down tightly enough.
If your coolant pump is leaking, it won't be long before it goes too. When you see a pronounced amount of coolant on the driveway, then it's time to get it replaced.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car