I noticed your post and want to answer your question.
Never fear, your problem is easily solved. The High Power fabric that you have on your screen has a special coating on the front of it that allows for the screen to be cleaned if necessary. All you need to do for light dust marks is use a vacuum with the dusting brush, if that does not work or if the marks are more than dust then you can use a mild soap and water solution. Just make sure you use a very soft clean cloth to clean it and don't use an over abundance of water (or it will streak). The fabric you have is very forgiving and I am convinced that you will be able to get the drywall dust off of the screen with either of these methods.
Good luck and enjoy your screen !
Blake Brubaker, CTS
Vice President of Sales
Da-Lite Screen Company, Inc.