You can have a sun-sensitive rash on your arm, for sure. Check out Mayo's article on Polymorphous Light Eruption for an example of a sun-sensitive rash you can get on parts of your body: http://www.mayoclinic.com/health/pol...uption/DS00911.
But you can't get a malar rash on your arms. A malar rash describes where the rash is, not what the rash looks like. "Malar" means "on the jaw or cheekbones," and thus a malar rash is, by definition, a rash across your face. You can check out wikipedia to read a good, basic definition of a malar rash: http://en.wikipedia.org/wiki/Malar_rash.
I know that doesn't completely answer your question, but I wanted to highlight the difference between malar rash and photosensitive rash.
Also, if this rash continues, I'd definitely go see a dermatologist. If it persists, they can biopsy it and get a better idea of what the rash is.