Tentative ID. I'm unclear if the species name is correct.
This is the only Ruellia I can find listed for the area.
It's appearance largely agrees with the entry on page 369 in Powell 1998 but seems divergent as it seems much smaller with far less elongate branches, suggests that the sexual organs should be visible and the description does not mention the white markings.
Growing a few miles from a Lophophora williamsii var. echinata population in Presidio County, Texas.