If you are working with acrylic, never leave it unattended. Lasers have been shown to create a particularly flammable environment when cutting acrylic, in whatever form it may take. If you are not present when your laser is operating - regardless of the material - you should not use it.
The type of acrylic you use for your application should be considered carefully. Extruded acrylic is best for laser cutting, while cast acrylic is best for engraving.
Remove the backside reflection by elevating the acrylic with Epilog's Pin Table or another support.