Duplicate
The Maitre (tall Greys) are addressed within the existing comprehensive answer on Grey alien types. That answer covers the Maitre specifically as one of the 165+ Grey species documented in the corpus, including their role, physical description, and relationship to other Grey sub-species.
The key Maitre content appears in the Grey types transcript series (S-101, S-102) and the general hybridization discussion (347).
See also: What are the different types of Grey aliens? (covers all Grey sub-species including the Maitre in detail)