They can use Mary. It just has to be under the form of Dr. Mary Agana
(as she's meant to appear in the Icons house).
Also, Legendary Truth isn't an IP. It's an original concept. Bloody Mary isn't an IP - Bobbi Weiner has just had a copyright in terms of certain usage and imagery of the myth.
That's like calling La Llorona an IP (in the way it came to us in HHN 23 - obvs could be an IP NOW).