this sounds like a filamentous green algae, probably one of the blanket weeds. This is not uncomon especially when freshly filled. I find after a couple of weeks it goes, however if there are lots of nutrients in the water then it can becoe a problem. you are able to buy products to get rid of it. I tend to twirl a cane in the water to collect it, but be careful you dont puncture the liner.

If it is spirogyra (If you spell it like that), which is a fine filmy filamentous growth that grows very quickly I would leave it until your taddies grow legs. While they are young and vegetarians they love it. If it gets out of hand ask at your pond supplies shop - they sell something non chemical to control it but getting senile as I am I can't recall what it is.

Thanks for the advice, I'll wait for a while before I get rid [or try]. I was hoping it might be some pond-weed that had materialised, but I seem to remember that something like this happened before; it was vertical blind cleaners, strapped onto the end of a cane that was the most successful remover; we used to have competitions!