I have a white Mollie fish that seems stressed. Yesterday she had a white cloud coming out of her and a long redish brownish string coming out of her also. Could she be pregnant or just ill?

The brownish string is usually poop that is just hanging on waiting to break off.

Not really sure what you mean about the cloud.

As far as could the Mollie be pregnant, it's possible. As far as fish go, they are among the easiest to breed and it will often just happen... provided, of course, that there is a male in the tank as well.

Mollies can actually store sperm from male fish for several months, so it is very possible that even if you have no males in the tank, she may have been pregnant when you got her. She may even have several batches without any further aid from a male fish.