Hi,
Substrate contact reduces usually the Eddy Current within the substrate with low resistivity and increases the quality factor of the inductor. However there are some important layout considerations when placing the substrate contacts. For example, the substrate contact should not form a closed loop around the inductor, otherwise your inductor will be asymmetric (S11 != S22).
Your PGS should be connected to the ground to isolate your inductor from the substrate.
The area under the inductor is white, because he has just eliminated the dummy generation, I think (but I'm not sure). In this way, you don't have dummy metal structures or p-wells under your inductor, that may degrade your quality factor.
please refer to papers from Burghartz or Tiebout, they are the best.
good luck