One reason I can think of is the fact that for the cheese slices that I occasionally buy, 41/50 cals come from fat. But for an overweight person (me) trying to lose weight and eating healthy+working out, can half a slice of processed cheese slow down results?
No food is unhealthy if it fits into your macros. A slice of cheese contains approximately 113g calories of which 9g are fats and 7g are proteins. Since you're an overweight person, you don't need a lot of fats, but proteins. Considering a slice of cheese provides more fats than proteins, it will hinder your long-term goals.
A far better substitution would be, say, 3 egg whites. It would provide you with 51 calories, has negligible fats, and 11g of proteins. This will provide you with healthy proteins that will keep your fat gains to a minimum while building muscle effectively.