Condensation issue

It’s most likely condensation. When the surface temperature is lower than the dew point, water vapor in the air will condense on the surface. Therefore this will happen on high humidity days when the slab temperature is cooler than the dew point. This is tough to fix without lowering the humidity in the space or raising the temperature of the slab or air. Moving air over the slab will help by causing the condensation to evaporate.