In general, "source available" or "shared source" are terms that will lead to less confusion in situations like this.
In terms of proprietary licenses, Aseprite is very good. It's one of the very, very few proprietary software products that I allow myself to use while developing games. But calling it open source is at best going to spark some disagreement/confusion, and at worst going to spark some needless debate from people (like me) who believe that the FOSS community needs to more closely guard how those terms are used and abused online.
Note that the main Aseprite dev also doesn't use the words "open source" when describing its changed license: https://dev.aseprite.org/2016/09/01/new-source-code-license/