The words I want don't yet exist. So I set out to find some myself:
Mentally Different
This one fits two of the three criteria: it's not too long, and it carries no negative connotation. I've used it before, but there's one major problem: inexactness. What, exactly, is mentally different? It could be anything, not just high-functioning autism! Time to try again…
Aspie
I didn't make this one up, which is good because I don't like it much. It sounds silly, and while it's short, its root is Aspergers' syndrome, which definitely has a negative connotation. (I don't like having a syndrome. It sounds dreadful!)
Neuroatypical
Not too long, not too silly…this one is turning out well. It's not very exact, but since autistic people call "normal" people neurotypical, it's more "connected" to autism than "mentally different" is. Then I looked it up…and found out that according to Urban Dictionary (a notoriously unreliable source), it means someone with autism or a whole host of other disorders; schizophrenia, bipolar disorder, dyslexia, et cetera. Another inexact term!
Nonallistic
The negative of allistic, which is a synonym for neurotypical– someone not on the autism spectrum. If allistic means not autistic, nonallistic must mean autistic. And since we already have autistic to refer to the severe forms, I claim nonallistic for my side of the spectrum; the high-functioning autistics. (I made it up, so I get to decide what it means.)