fine particle; granule
Refers to very small solid particles, often in technical or scientific contexts, but also used generally for tiny grains or specks.
This filter removes fine particles from the water.
Pollen is a fine particle that floats in the air.
Compound of 微 (bi, 'tiny') and 粒 (ryū, 'grain'). The reading is on'yomi, and the word is a straightforward Sino-Japanese technical term.