It isn't any of the ones quoted on Tom Volk's page of fungi commonly occurring on potted plants he Tom Volk's Fungi (to find it, click Fungus of the Month, then near the top click the "Fungi growing on your mulch"). Leucocoprinus birnbaumii is the most common culprit, but not in your case.

It does rather remind me of a Peziza (Cup fungus), and indeed Peziza cerea - Wikipedia, the free encyclopedia or Cellar Cup seems faintly possible, tends to grow on man-made piles of rubbish. (There are other kinds of cup fungi in the Pezizales group.)

But I have half a memory of there being something else that looks rather like that, but I'd have to hunt through my fungus books for it some other time.

fwiw, it is unlikely to be something you'd want to eat. It is probably contributing to the breakdown of the constituents of the compost, thus releasing the compounds that feed your plants.