If your plant is in an environment that doesn't get below 60F, you really don't need a seed heating mat. Once seed sprout, the mat offers little benefit in the majority of circumstances.
I'd nix the mat (as per the Cornell sheet warm soil and cooler humid ambient temps cause the problem) and, as Patrick suggested, reduce the watering. Wait until the plant looks like it's starting to wilt before you give it water.
FWIW,
Bill