Ducks somehow find water. They eventually visit most home garden ponds that they can fit into. Duckweed, Lemna minor, is likely to come with them. It adheres to waterfowl and other wildlife for that purpose. It proliferates very efficiently, and almost typically becomes a nuisance. Eventually, proliferation in a healthy pond should stabilize to a tolerable rate.
Individual duckweed plants are tiny. Their oval leaves are typically less than a quarter of an inch long. Each floating leaf extends its single root less than three quarters of an inch into the water below. Plants produce no more than four rooted leaves before dividing into a few smaller plants to repeat the process. Bloom and subsequent seed are uncommon.
As a floating aquatic plant, prolific duckweed might obscure koi and submergent aquatic plants within garden ponds. However, it also helps stabilize healthy aquatic ecosystems. In fact, it is useful for bioremediation of agricultural and industrial applications. It absorbs detrimental substances from water, while producing fodder and biomass for composting.