Mark, that blue tarp isn't enough of a barrier to break the heat transfer which will be very important given the location. You need something thicker. Even sheets of styrofoam or insulation board will be better or it will be a considerable issue.
Yes, from the slab. I would highly recommend something that breaks the heat transfer and a tarp isn't going to do it. Even if it's in the shade, concrete around the area will transfer the heat to the pool. A concrete pad is like a huge heat absorber and will slowly transfer it to the pool. You've got to break the transfer.