Can the algae in the pool be caused by one mallard duck or is it a bad filter or maybe not enough chemicals?

THe answer to your question is...

Yes.

It COULD be any one of the 3 but I'd bet on a combination in 1 way or the other.

Remember, it only takes one microscopic spore to turn a pool into green slime in 24 hrs given the right conditions.

Good Luck! Twig CPO/minor pool & spa god