For sale is a large clump (see photos below) of Star/Ivy leaf duckweed (Lemna trisulca, more info here). This plant is a floating plant but unlike the common duckweed (Lemna minor, see photo here), the Star duckweed is grown completely submerged under the water surface (see photos below). They are slow growing compared to the common duckweed. Each frond will divide and multiply into a large mass (much like Java moss) interconnected by stalk-like structures but can be separated easily. Each frond bears a single short root. My Tiger and Cherry shrimps just love to pick on debris trapped on them! It also provides hiding places for small fish too.
See my tank setup here and more photos here with very healthy fish and shrimps (all disease-free!!) and I do not add any medications to my tank but I do dose plant fertilizers (mostly from Seachem). It is advisable to rinse and inspect for snails before putting them into your aquarium. Below are photos of the Star duckweed for sale: | 
