It's a lime buildup on the rubber seal and the surface it seals with, the dome shaped flapper that moves with the foot pedal. Take a kitchen scouring pad (don't use anything metal like an SOS pad) or scour side of a sponge and very lightly rub both surfaces. You can use a little vinegar or Lime-A-Way, or CLR. For me just a very light rub with the scour side of the kitchen sponge is all it takes.
From what I could feel on mine the first time I cleaned it, the buildup was more so on the bottom side of the rubber gasket than the plastic flapper that moves.
There are 10 types of people in the world.
Those that know binary, and those that don't.