On bright sunny days bass will flock to areas like this in great numbers.
Very often the edge of a mat will be right over a drop off.
Cast a plastic frog beyond the holes, drag it over the mat for a long distance and then swim it through the hole. If the fish don't hit it before it gets to the hole, very often it will get nailed once it hits the water.
When possible, keep the boat outside the mat to prevent spooking any fish swimming underneath it.
Remember that the outside edge of a line of scum can sometimes indicate a drop off. that is NOT the case here. The scum is formed because there are farm fields nearby and fertilizer from the fields finds it's way into the lake. This helps some of the plant species grow. Left unchecked, this scum would cover the entire lake if it were not treated with other chemicals.