hmmmm...two separate but related issues here. The black sooty surface is sooty mould, which will be growing on honeydew that is in turn excreted by sap-sucking insects.
The white grubs are the sap-sucking insects in question, possibly scale insects (or maybe the closely related Mealy Bugs) and their fluffy egg masses from your description - Google "Scale Insects" and see if that looks like what you have.
They are highly resistant to insecticides, but are also immobile, so can easily be removed with a sponge & some soapy water (use vegetable based soap like Ecover to avoid possible damage to the leaf & soil). Be careful to collect them (in a bowl etc.) as you go.