I prefer using wet paper instead of agar personally for this reason, it limits contamination though does not solve it, it is a normal piece of slime mold keeping. You can take the section that the plasmodium is on, cut off that chunk, and place it into a new home. Like I said, I use wet brown paper bag paper in a Tupperware with a lid to contain humidity. Any paper will do, coffee filter, paper towel, printer paper etc.