The Mormons started Migrating to California in 1848 and it’s conceivable one might have stopped to carve this on the rock.
Problem is, it ain’t Hebrew or even some forgotten language.
You are aware that not all the jews left Spain. In fact, some analysts say more Jews stayed in Spain and "converted to Christianity" than left Spain.
Their culture and knowledge didn't just disappear in 1492. We are talking about 1 or 2 generations into the discovery and re-settlement of Mexico, and there should have been plenty of people who'd learned Hebrew at home in Spain out there wandering about looking for all the world like any other Spanish caballero ~ horse and all.
By 1600 the Spanish had effective control of just about everything they'd ever heard about or seen in North America ~ so you'd had educated people leaving their marks.
The Church of the First Born (not at all related to the Mormons) began migrating to this area and others throughout the West when it was still Spanish and Russian territory.