Viewed from the Northern Hemisphere, this is a pictograph of a first quarter moon with the addition of some rays of light. Hence, it is an ideograph for the noun moonlight.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1695 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 18: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 2157 (Moonlight): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.