The common name in the US is "duckweed". From a distance the stuff appears
as a pale green, non-reflective, uniformly colored scum on the surface of a
pond. Up close, there's small individual circular leaves about a
millimeter or two in diameter, with one or two fine hairlike roots
underneath. It's very common here in the upper Midwest. Since plants can
be transported on the feathers and feet of waterbirds, it's probably
growing within an hour or two of nearly everyone on the list.