Food Safety – How Dangerous Is Mold at the Bottom of a Container With Fat?

Well, first off, mold grows from spores, and your Pyrenean cheese was likely already "contaminated" with Penicillium roqueforti, Penicillium glaucum, and Penicillium candidum spores at the cheese shop (it'd be surprising if the cheese shop isn't covered with them!). So, if you keep it in a environment habitable to them, they will grow. I suspect that you'd get the same results even if you put them in two different plastic containers.

As to its safety, if it was indeed one of the used-in-food Penicillium molds, it should be safe to eat—you can make sheep's milk blue cheeses (e.g., roquefort) after all. But the problem is that the same humid environment that encouraged the Penicillium colonization also encourage other molds—many of which you don't want to eat. Also, I'm pretty sure there are blue-green molds you don't want to eat.

Per NC State's Extension's article on pickles and sauerkraut (some emphasis added):

Pickles or sauerkraut mold during fermentation.

Answer:

Unsafe—microorganisms are growing improperly.

Possible reasons

• Fermentation temperature was above 75°F.
• Too much salt was used, not allowing adequate lactic acid production.
• The cloth on top of the kraut was not kept clean during fermentation (may need to be replaced after skimming).

Per Penn State Extension's Saurkraut Guide (emphasis added):

Do not taste it if you see mold on the surface, feel a slimy texture, or smell a bad odor.

Their PDF on Saurkraut says:

To avoid surface mold growth, keep the cabbage submerged at all times by covering it with a plate just small enough to ft inside the fermentation container or with two or three clean quart jars filled with water. An acceptable alternative is to fill a large, sealed, food-grade plastic bag containing 4½ tablespoons of salt and 3 quarts of water.

Again, no indication of acceptable types of mold.

I cannot endorse the existing answers which indicate that mold is an acceptable part of the saurkraut making process, unless credible science based evidence is presented, from reputable sources such as University Extension Centers or, better yet, peer reviewed journal sources.