A while ago I had access to a 60W Epilog CO2 laser cutter. The local wisdom was that the top paper should be left on the acrylic because the beam caused the plastic boil and leave a spatter of dots on the surface. I suspect that the white stuff you are seeing isa build up of that spatter. Please note that I have yet to try cutting acrylic with my 2.5W laser.
Also, I prefer the look of many repeats of a fast cut to fewer repeats of a slow cut. I have been working with CX board from a local framing shop. This is white faced, solid cardboard about 1.3 mm thick.
FR 600, Power 255 Passes 20 - clean cuts
FR 600, Power 255 Passes 12 - mostly cuts
FR 210, Power 255 Passes 7 - mostly cuts.
At 600 the cut line is narrower and there is less charring of the edges.
Another consideration is the time between cuts. Again, experience with the Epilog showed that it was necessary to give a cut time to cool down before taking a second pass or ther was a significant risk of the material catching fire. Thus, I am also experimenting with making sure that there is a long toolpath between passes.
That's it for now.