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”.
A correlative appearing in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/111), a combination of ᴹQ. uma- “some” and the locative suffix ᴹQ. -sse.