It's a diminutive of
Maria, and the -k- makes it not as much like a double diminutive but more like baby talk, and it can be considered rude and/or condescending when it's not used affectionately/for a family member. So here it would be OK. It's just
Masha only a little bit more diminutive, but not as much as Mashenka or something like that.
----------------------------------------------------------------------------------------------------------
They told me I was gullible... and I believed them.