What is the smallest of all flowering plants that appears as minute green grains floating on the water?

Watermeal is known to be the smallest flowering plant and appears as tiny green grains on the water's surface. It is a free-floating aquatic plant that is part of the family Lemnaceae, which also includes duckweed, but watermeal is significantly smaller than any duckweed species. Each individual watermeal plant can measure less than a millimeter in size and typically forms dense layers that can cover water bodies, giving them a green tint. This unique characteristic, along with its reproductive strategy that allows it to thrive in various aquatic environments, distinguishes it as the smallest flowering plant found in water.

In contrast, while duckweed is indeed a small floating plant, it is larger than watermeal and often visible as small green discs rather than grains. Cattail and Elodea are both much larger plants and do not float in the same way, with cattails being emergent plants and Elodea being a submerged aquatic plant. Thus, the identification of watermeal aligns directly with the description provided in the question.