There are plans to fence off Collier’s pond, though still allowing limited access. The weed carpeting part of the pond is not Crassula but another invader: red stemmed parrots feather (Myriophyllum brasiliensis).
Closely related to Myriophyllum aquaticum, which is on the Schedule 9 list of invasive species, M. brasiliensi is still available for sale as a pond plant. Don’t buy it because it can escape from your garden! Efforts will be made to keep the red stemmed parrot’s feather under control.
There was great tadpole activity on the edge of the weed when the photograph was taken.