marmalade is from French marmelade from Portuguese marmelada from marmelo "quince", from Latin melimēlum. The first /l/ in the Latin word became /r/ in Portuguese by dissimilation - because there is another /l/ later in the word.
Latin melimēlum is from Greek melímēlon "kind of apple grafted on a quince", a combination of μέλι méli "honey" and μηλον mēlon "apple". Greek μέλι is from Proto-Indo-European *melit- "honey" (IEW meli-t 723).
mildew is from the suffixed zero-grade *ml̥d-to- "honeyed", which in Proto-Germanic combined with *đauwaz "dew" (from *dheu-² "to flow") to form *meliþ-đauwaz "honeydew". This became Old English mildēaw, meledēaw "honeydew, nectar", then mildew.
Melissa is from Greek μέλισσα mélissa "honeybee", from the suffixed form *melit-ya.
source: The American Heritage Dictionary of Indo-European Roots, Calvert Watkins, 2001
The Oxford Etymological Dictionary, 1991