Is this mold?

Relying on an answer you get on here would be a huge mistake. Put your hand in your pocket and get an expert to look at it.

I did not look at at the picture - but did you hire a home inspector? Everyone should have a home inspector go through a house prior to making your purchase. I understand rules can vary by state to state (or country to country depending if you are outside the US) - but I know in my state, when you sign the agreement to purchase a home - it is always pending anything coming up in a home inspection.

It is a huge investment - why not shell out the small amount in comparison for a licensed home inspector.