Incuse is an adjective that refers to a design that is recessed or "cut into" a surface.

Incused is the past tense verb form of incuse.

I'm not sure how it was first misused in the chip hobby, but most everyone today uses "incused" as an adjective. The proper form is to refer to an "incuse mold", or "incuse lettering" (or "non-incuse" if you mean a design flush with the surface).