An Etymological Dictionary of the German Language/Annotated/Schimmel

Schimmel, masculine, ‘mould,’ from the equivalent Middle High German schimel, masculine, for an older *schimbel, Old High German *scimbal, which may be inferred from the Old High German derivatives *scimbalên, ‘to get mouldy,’ scimbalag, ‘mouldy.’ The Middle High German form is due to a confusion with schîme, masculine, ‘glimmer’; compare Dutch schimmelen. Old High German *scimbal has no corresponding form in the other Teutonic dialects —

Schimmel, masculine, ‘white horse,’ late Middle High German, identical with Schimmel, ‘mucus.’