New Zealand Pygmyweed Crassula helmsii AKA Australian Swamp-Stonecrop
Found in a Lagoon with seeds remaing stertile the best way to remove this is by raking as it spreads vegetativly
This is an Invasive Plant under Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild.
More here NZ Pigmyweed
NZ Pigmyweed whats happening elsewhere Potteric