Is This Algae Or Watermeal / Duckweed?
Watermeal and duckweed are commonly confused for algae. In the picture below, there is actually NO algae. The pond is covered with an aquatic plant called watermeal which is not rooted and floats on the surface of fresh water ponds.
Algae is easy to distinguish from aquatic weeds because it is either stringy and mat-like (see below) or is microscopic and gives the water a green tint.
Watermeal and duckweed look like green cornmeal, with round particles that can be seen with the naked-eye
See also: How to get rid of aquatic weeds?