yeah, the tank watch uses 3 or 4 rods with floats and magnetic switches. I guess the tank was full, turning on the red light (float high on its rod). then when you pumped out, the float that activates the green light, in the low position, went on as it should but the red is still stuck high.
you may want to try tapping the top of the gauge assembly as that could free the float. otherwise, you have to pull the gauge assembly and clean the damn thing...
been there, done that. not a lot of fun but no worst than changing a diaper if you've ever done the kid thing.
of course, that's assuming that the tank and gauge was installed with enough space on top to pull the assembly out!