Im not certain but as Taz said it may be duck weed but it maybe small bits of frogbit. They look the same for about a day when frogbit is small but then it starts growing fast. Either way if you dont have many plants in your tank either of the 2 are nice plants. I have regular duck weed (minor) and greater duck weed( looks neat cuz it gets a red coloring to the bottom of the small leaves and it grows to about 1/2 wide), and frogbit in my indoor pond, 10 gallon and a 30 gallon. All nice plants.