A singular indefinite adjective “some” in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/105), also usuable pronominally to mean “something” (PE23/105). This adjective is specifically singular, as in uma elda “some (unknown) Elf”. An indefinite plural would use the partitive plural suffix -li: eldali “some Elves”.
Neo-Quenya: In Tolkien’s later writings ma was the pronoun for “something”. However, I would retain uma for the adjective form, and also use it for a singular indefinite subject since ma might be confused with an interrogative: uma tíra me “something is watching us”, as opposed to ma tíra me which might be interpreted “what is watching us”.
The prefix for singular “some” in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/99), used in formations like ᴹQ. umallume “some time” and ᴹQ. umas(se) “✱somewhere”. It is probably related to the interrogative root √MA, which is also the basis for (later) ma “something”. See the entry on correlatives for further discussion.