A word in Declension of Nouns from the early 1930s glossed “grit” with various forms representing the inflections of nouns with lost ancient vowels: malǝ- (PE21/19, 24). It may be a later iteration of ᴱQ. mar (mard-) “grit, course grain or powder” from the Qenya Lexicon of the 1910s, a derivative of the root ᴱ√MṚŘṚ [MṚÐṚ] “grind” (QL/63). Later still Tolkien gave the primitive form ✶smalŭ with the gloss “dust, grit” in Common Eldarin: Noun Structure (EVS2) from the early 1950s, though this primitive form had no derivatives.
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer to use ᴹ✶smalu with its 1930s sense “pollen, yellow powder”, as this form has derivatives in The Etymologies (Ety/SMAL), and its root meaning ᴹ√SMAL “yellow” is a better match to the later root √MAL “yellow, gold”. I think it is possible to retain ᴹQ. mál “grit”, however, by assuming that it is a derivative of ᴹ√MBAL “✱pound”, which is a better match with 1910s ᴱQ. mar < ᴱ√MṚÐṚ “grind”.
A noun in The Etymologies from around 1937, glossed “pollen, yellow powder” and derived from the root ᴹ√SMAL “yellow” (Ety/SMAL).
Conceptual Development: In Tolkien’s earlier writings, ᴱQ. malo was “rust”, first appearing in the Early Qenya Grammar and Early Qenya word-lists of the 1920s (PE14/42, 44, 71, 74; PE15/77; PE16/111) and later as ᴹQ. malo “rust” in the Declension of Nouns of the early 1930s where it had a stem form of malu- (PE21/12). Most likely these earlier forms were based on the early root ᴱ√MALA “yellow” from the Qenya Lexicon of the 1910s (QL/63), though in QL itself the word ᴱQ. malo was “moth” from the distinct root ᴱ√MALA “crush” (QL/63).
Neo-Quenya: For purposes of Neo-Quenya, I think the “pollen” and “rust” meanings for malo (malu-) can co-exist, with both originally considered a form of “yellow powder”.