@primal_healer is right and it looks like damage from too much heat/flashback. (Flashback is when the laser hits the tray causing it to heat up and burn the back of the material.) Mirrored acrylic is super finicky, but there’s more than one way to get good results. For 1/8" acrylic I cut with the reflection facing up and a speed of 140 and 85 power. I remove any plastic masking and put regular paper masking on the back (non-reflective) side that rests on the tray to prevent flashback. Another option is to place the acrylic on top of a little cardboard or scrap material so it’s not sitting on the hot metal tray.
Other setting work too and there’s been lots of discussion about it. Here’s one. 