Increase feed rate a bit. With that type of pass in acrylic you want to be getting chips that are quasi snowflakes to proper chip form. Dust is a sign that you are cutting to slow and you will deal with melt and it will cause problems. Next is this cast or extruded acrylic (easiest way to tell that I know of is almost all extruded has a translucent protective covering and cast is white) When I am surfacing 1212 I am going between .75 to 1.5 mm deep with a 3 flue TCT MG end mill and running it at about 14K and 2.3 m/min and getting rather nice finish. Acrylic is a problem child in that it will melt easily IF F & S is to slow (even cast will start too but requires really slow).
Cutting plastics requires testing and learning what works, due to large number of variables that are out there. Type of plastic, type of end mill, S & F's; these all play a part then add how rigid is the machine and things get interesting.
Michael
PS From everything I have seen I agree with ADCNC that is redeposit of plastic on passes after the original cut due to too slow of feed rate.