The etymology of Mallor's name is hard to work out, although, it is likely that at least part of relates to the Sindarin word mall- meaning "gold".
Sindarin
mallor
masculine name. Mallor
Elements
Word Gloss MAL “gold, yellow, gold” glaur “gold [light or colour]”
Mallor
Mallor
The 3rd king of king of Arthedain (LotR/1038), perhaps a combination of the root-form √MAL “yellow” and the suffixal form -lor of glaur “gold(en light)” (as suggested by David Salo, GS/355).