Is this mold?

[Cindy Joseph]

I bought my first place which is a duplex from 1957 in December. I had an inspection before I brought it and they tested for humidity and thought there was a low risk of mold. I bought a Hydro meter which mostly reads 50 to 60%. Sometimes it shoots up higher when we shower, we do not have vents yet. A few days ago I saw some black residue on the corner of the bathroom wall (see picture). I scrubbed it off and peeled some of the paint, underneath, there are some lighter gray residue. Is this mold or moisture? Should I get a professional to look at it? I bought it at home test kit also.