It's a rule of thumb: The softer the cheese, the more you need to be concerned about mold, cheesemaestro says on Chowhound. Bacteria easily spreads through soft, unaged cheeses like cottage cheese and fromage blanc, ricotta, and fresh mozzarella and goat cheeses. The same is true for grated cheese, no matter how hard it was in brick or wheel form. When any of these get moldy, throw them out. In contrast, bacteria grows much more slowly on hard cheeses—simply cut off the moldy parts and you'll be fine.
Consider the mold's color. White and green mold are generally safe—cut it off and the cheese should be perfectly OK, sunshine842 says. But pink or orange mold—more specifically, mold that's Tang orange or bubble gum pink—produces highly toxic by-products with deep roots in the cheese. "Anything with pink or orange goes straight into the garbage, no questions asked; no debate," sunshine842 says. Delucacheesemonger agrees, since it might indicate the presence of the bacterium Serratia marcescens, which is very toxic. Black mold can also indicate the presence of toxic organisms.
There's one important exception to the zero-tolerance rule about reddish molds, cheesemaestro notes. "Washed rind (stinky) cheeses usually have a reddish or orange rind colonized by a bacterium (Brevibacterium linens) that is perfectly safe and should not be confused with red or orange mold."
Also, mold on the outer rind is much less of a concern than mold in a cheese's interior, or paste. Amazingly, living cheeses like Brie may actually form a new rind along the cut edge, and this is a good thing; it blew sunshine842's mind the first time she saw it, but there's no need to worry.
Finally, any obviously bad smells, like ammonia, indicate a trip to the trash can is in order, sunshine842 says. The cheese may not kill you, but it'll definitely taste awful.
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