Firstly, if there is toxic mold (the kind banks make agents and buyers sign release forms for on bank owned homes) I would look at a professional remediation team. They do amazing things. The big point here is that homes with mold should be the cheapest thing in the area.
With that said, hopefully you're still in your inspection period and can make an issue out of it. If you're buying a bank owned home, they probably won't do the remediation but it never hurts to ask. They may agree to a lower price though since they know they'll have to deal with it regardless of who they sell to.
If you're dealing with a traditional seller and you're still in the inspection period, you have a lot of power and should use it to your advantage. If you're past the inspection period or didn't submit an offer with that contingency, you really need to consider your options. How bad is the mold? What will it cost you to do a full remediation? How much earnest money is at stake here? Answers to these questions will help you determine the best path.
I work with many investors so I have a great mold remediation team if you're interested. Feel free to email me at firstname.lastname@example.org or call at 612-849-9079.