Mold or mildew?

I ventured up to my attic hadn't been up in a long time and saw this. Is this mold or mildew? I really don't see much evidence of this anywhere else in my house other than in my bathroom around one of the AV vents (and it's way less noticeable than the attic). What should be my next steps? The house is very old (1938) so the insulation is definitely not very good.