Need to know if this is mold?

There is no way for us to tell for sure. It looks like mold to me and that's a strange place for dirt to just accumulate. So lets play it safe. Get a spray bottle of white distilled vinegar and spray the area thoroughly. The vinegar is great for killing not only surface mold but also going below the surface to the roots. Let the vinegar sit for an hour and then wipe off. You might need a second application. You can then rinse with hot water and let dry. Spray again and let the vinegar dry naturally, the smell of the vinegar will dissapate in a few hours. Don't use bleach, it's only good for surface mold.

Spray bleach on it ;it will pretty much disappear if it is mold. Assuming it is not so thick that one application of bleach is insufficient . There will clearly be a difference in appearance although a few applications of bleach may be required.