I have a question for you. Do you just drop the mouse in the enclosure right on top of that substrate? If so, do you ever have issues of him swallowing a lot of that substrate when eating the mouse? I ask this because I have the exact same substrate for my bp and decided to feed him in a different container so he wouldn't swallow a lot of substrate.
Substrate ingestion is shouldn't be an issue. Whatever is stuck on the mouse is not enough to cause problems.
If you choose to do the "drop the f/t in the enclosure" method, you can lay down a piece of paper towel and just put the prey item on that.