The Greys! We assume you're aware that the term "grey alien" or "greys" is commonly used to describe extra terrestrials (though there is some dispute whether grey aliens are actually grey). So, we think you may be correct that this may be too much of a general -- or generic -- term to qualify for a trademark for your dolls (and no, your copyrights won't help).
The Examiner's Response. If you seek to register the trademark, a trademark examiner might consider the term to be merely descriptive (generic). That is, the term "grey aliens" is used to represent a type or class of doll product and does not distinguish your dolls from others in that class. If that is the case, you'll receive a substantive rejection for your application and you'll need to convince the examiner that consumers identify the name with you and not with a class or type of product. Alternatively, an examiner might consider your mark to be weak (or descriptive) in which case you would need to demonstrate more than five years of sales or other evidence of consumer associations. It's also possible that your application might fly through the USPTO without objection (that occasionally happens). One suggestion to improve your chances is to consider personalizing the term, for example, "The Grey Family" or naming the aliens, for example, "Dorian" or "Jennifer" (and don't forget the alien dog doll, "The Grey Hound").