Assuming the grease/scum floats and is not "contaminated" with a bunch of particulates, its specific gravity would be somewhere between 0.85 and 0.95. Given that, if you have the volume, you may compute the weight by V*specific gravity*unit weight of water. As an example, if you have one cubic foot of grease, its weight would range from 53 lbs to 59 lbs. Please note that this value includes entrapped moisture in the grease. To get an actual "dry weight" you will have to know the moisture content of the grease.