Ducks' legs are kind of fragile - they can get hurt fairly easily. One type of injury is splay leg, it happens to ducklings if they are on a slippery surface. It can be corrected, but to keep it from happening, ducklings need to be on something that is not slippery.
They can also get alot of bumblefoot if they keep getting little cuts and scrapes from what they walk on. That's why concrete is not so good for them.
Is the plastic slippery, or does it have holes in it? Something else that can hurt a duck's feet and legs is if there are holes they can get their feet stuck in and turn their ankle or break a bone.