What your seeing is the beginning of corrosion on your water lines and over time it will corrode and cause a small drip or small leak.
Q. Why does it leak only sometimes?
A. Often time the corrosion on a hot water line will seal itself temporarily because of the mineral build up plugging the pinhole...eventually rust will eat away the waterline or joint causing a water leak.
Call US! to have this Repaired by an Experienced Plumber.