Why is my pond / lake melting when the temperature is still far below freezing?
My guess is that since ice (and the liquid water underneath it) is much better at absorbing thermal radiation than air, then even if the air temperature is below zero, the ice can heat up in the sun and melt a little bit. It will presumably then refreeze at night when there is no sun to keep it warm.

This was probably also happening on the surface of the snow as well before you cleared it but you didn't notice it because there was no colour change.