Yep. It's mold. How toxic it is depends on how much media hype you believe. If you already suffer from some health issues, then it could cause additional problems. Your landlord needs to be persistant and figure out the problem. The mold is an indicator of something that is going to cost him even more in the long run. If it's damp enough for mold, it's going to rot his house away.
You probably have a lot of protection as a renter, particularly from mold.
I know in my area, that if mold is found, the owner is responsible for setting up & paying for temporary residence, moving all of your belongings, repairing the cause, mitigating the mold and subsequent inspections.
I'd get a hold of your states Department of Health and your local renters association.
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