、。
A single tear fell from those eyes.
Literal
Those eyes [from-から] one-grain [genitive-の] tear [subject-が], flowed-fell.
Literary and evocative. 一粒 uses the counter 粒 (つぶ, grain/drop) — a poetic choice for tears, emphasizing the solitary, precious quality of a single drop. 流れ落ちた ('flowed and fell') is a compound verb: 流れる (to flow) + 落ちる (to fall), painting a slow, cinematic image. その瞳 ('those eyes') uses 瞳 (ひとみ), which specifically means 'pupil' or 'eyes' in a literary/romantic register — distinct from the everyday 目.