How to translate "profesor auxiliar" from Spanish to English?

In the US, the usual title for such a position is Adjunct Professor.

See What are the roles and responsibilities of an adjunct faculty?

At many universities where the professors (even the adjuncts) desire to have that phrase restricted to those teachers who have earned a terminal degree, a more common title for non-tenure-track teaching positions would be Instructor or Lecturer.

NOTE: this distinction is more common in disciplines in which the terminal degree is considered more of an absolute prerequisite for a tenure-track position; in many, more "skill-oriented" disciplines (I'm thinking music performance, theater, art, and yes, often computer science), such an academic degree is indeed considered...well, "academic," and a master's-level teacher with a plentiful level of "real-world" experience can indeed be considered sufficiently "expert" to be granted the exact same level of professional courtesy. Many schools can still be found to have (grandfathered in) veteran master's-level teachers who were in fact granted the luxury of a tenured position (whether dubbed Instructor or Professor).

P.S. It also bears asking whether the "Master's" degree in your country is comparable to the Master's in the U.S., or whether there is a level beyond it in your country. If there is no higher level than "Master" where you come from, then I would argue that your Auxiliary Prof. absolutely correlates with Assistant or Associate Professor here.