I'm no expert in slime molds, but your specimens do in fact look a lot like the sporangia, or spore-producing reproductive structures (i.e., fruiting bodies), of a Mycetozoan slime mold in the class Myxogastria. Physarum and Comatricha are possible candidate genera based on structure of their sporangia that I can find easily online.
See below for an example image:
Sporangia of Physarum pusillum. Source: Wikimedia
I don't think I can provide a more specific answer given the quality of your photo or my general lack of deeper knowledge of these organisms. Regardless, no quick search for slime-mold infections in plants produced any information regarding slime mold species that regularly grow above-ground on cabbage.