You see the word for filamentous fungus written as both 'mold' and 'mould'. Which is correct?

We agree with Frank Odds' analysis of the etymology of these words. Mould is derived from the Norse mowlde, for fuzzy. Mold, on the other hand, is from the French molde, for shape or form. Thus, if you make gelatin in a mold, a mould might then grow on the gelatin in your mold.