Mega algae on plumbers' vinyl under steps

Based on some recommendations we read here and elsewhere we bought a shower pan liner aka plumbers' mat or plumbers' vinyl to put under the wedding cake steps in order to protect the pool's vinyl liner. This is a sheet about 4' x 6' of 40 mil strength, grey, made of PVC.

We put it under the steps and cut it to fit with a 2" overhang all around.

We've had these steps for 7 seasons now, never a problem with algae. We drilled some additional 2" holes into the sides to allow more water flow and circulation and have always been pleasantly surprised at the lack of algae buildup inside and under the structure.

Alas, the plumbers' mat seems to be an algae magnet! With no visible algae anywhere in the pool, not on the bottom nor on the sides or in the various little dips and CC always at 0 the top of the mat under the steps and the 2" overhang around the sides of the steps turned a deep, rich green! Brushing it swirled up a nasty cloud of algae which immediately translated into a CC reading. I super chlorinated and controlled the mess easily enough.

When I pulled the mat out from under the steps it was an appalling shade of green. Nothing on the vinyl liner under the liner, roddly enough. It all was growing on top of the PVC mat.

I do not understand this.

I like the mat because it's heavy and would seem to be an excellent protection for the pool liner under the steps, but why does it attract algae like this?

Weird. Sorry. I'm using a piece of shower pan liner and it gets no more yuck than the rest of the pool.

Perhaps it was contaminated at purchase?

Possible, of course, but it was packaged in a sealed plastic bag.