It's either residue from the tape (try some Goo Gone) or the paint you applied the tape to wasn't fully cured.  If it's residue it's easily removed with that product. If the surface of the paint has been changed it will have to sanded and recoated. You don't need to leave the masking tape on there for very long, just long enough for the coat of paint to become tacky. I usually remove the tape almost immediately. 

I recently had the same problem when applying a second epoxy coat in the build of a Passagemaker pram.   I had applied Frog Tape to map off areas where fillets were applied and this was followed by a second epoxy coat to the boats interior.   I did not have this problem when I used the 3-M Blue painters tape.  Needless to say, I will not use Frog Tape again and suspect this is due to an adhesive residue.

Pat Yeatts