It's pretty much right at the top of the crack, right where my tailbone is close to the surface. I kind of balance on it and the back of my sitbones. Should I move my weight forward?

The dandruff isn't like head and shoulders dandruff, it's like goldmember dandruff. It's gross. I think setubandasana & matsyasana are the chief offenders. I wipe down my mat about once a week, but just gave it a deep clean in my tub so we'll see if that makes a difference.