Board Thread:Naruto Discussions/@comment-24847491-20140921145042/@comment-1512913-20140922110152

Curse Marks are "instant soup" versions of Sage Transformation. Sage Transformation is Jugo Clan bodily fluids' reaction to Senjutsu chakra, so yes, it is Sage Mode.

EDIT: I'm yet to see Hashirama summon a slug or recognize Katsuyu, so I think you are wrong.