Hello! i've had a ~1 centimeter (flat) red area on my cheek for roughly 3 weeks. next to my mustache. first i thought it was a shaving rash. historically i don't have shave rash for more than a day. *i use low strength benzoyl perox for years daily?

Benzoyl peroxide should not cause the rash, especially you have used it for years. A rash which persists for 3 weeks is also not a shaving rash. Is the rash itching? How is its border and shape? Has its size changed in the last couple weeks?

The differential for a red patch on the face is very long. The fact that it has persisted for 3 weeks is not a particularly good sign. See a dermatologist for a proper diagnosis and treatment.