Eragrostis mexicana is one of the lovegrasses, a genus of very similar species, 16 in Arizona. Mexican Lovegrass is very similar to many of the other species, especially the invasive, ubiquitous species Eragrostis lehmanniana which is taking over some of Arizona's desert regions. Mexican Lovegrass won't be on the test--identify it at your own peril.
The lovegrasses of Arizona are mostly quite similar, but Stinkgrass, Weeping Lovegrass, Sawtooth Lovegrass, and Teal Lovegrass are easy. Stinkgrass has comparitively large, whitish spikelets. Weeping Lovegrass has extremely long (two feet long!) leaves whose ends curl. Sawtooth Lovegrass, of the deserts only, has roundish spikelets that are quite un-lovegrass-like. Teal Lovegrass is the only tiny one: 8 inches high or less.