Learn French: Cependant, la moisissure va en se développant sur la croûte. - However, the mold goes on developing on the rind.

This sentence uses the structure aller (conjugated here as va) + en + present participle (développant) to show that an action is progressing steadily over time. By using va en se développant, it emphasizes that the mold is gradually or increasingly growing, rather than just stating a simple ongoing fact.

Where does the reflexive pronoun se go when a verb is in the gérondif (the en ...-ant form)?

The reflexive pronoun always goes directly between en and the present participle. Because the base verb is the reflexive se développer (to develop), the gérondif form correctly becomes en se développant.

Could I just say la moisissure continue de se développer here?

Yes, you could, but there is a slight difference in nuance. Continue de se développer simply means it keeps growing without stopping. The structure va en se développant specifically adds a sense of gradual, continuous progression—like saying the mold "becomes more and more developed" over time.

I know croûte usually means the crust of a bread or pie. Why is it translated as "rind" here?

In French, la croûte is a versatile word for the hard outer layer of many foods. While it means "crust" for bread, it is also the standard word for the "rind" of a cheese, which is likely what the mold is growing on in this context!