2002 Pontiac Grand Prix Low coolant light on

My Low Coolant light is on all the time. The fluid is topped off in radiator and reservoir. I have changed the temp sensor that is off those in front left of car behind passenger headlight and I have changed the thermostat. The light stays on all the time and has been on for about 6 months now. Yesterday I took off cap and checked and I couldnt see any fluid so I topped it off and the reservoir was low as well and I topped it off. I started car and let run a few minutes and light went off but then came back on after a couple min. I rechecked the levels and they are still full. I also observed a little bit of sludgy mud looking grease on inside of cap as well as splattered down low in engine compartment and I'm not sure whats going on there. I had car running and checked for leaks nut noticed nothing. The car seems to run fine. Any ideas? Thanks