From the dry material on the outside it looks like you have a 5 gallon bucket of primer with about 1 gallon left.
You can see the paint is a bit broken by looking at the watery outside edges. It may be usable still though but it is not going to have the same properties as it was supposed to.
On the rust - not mold - well rust is a pretty damn good coloring agent. It is already in the paint. Probably the best thing to do is find the cleanest side, wipe that side of the inside down really good with paper towel to remove rust and dump this into another bucket that can be stirred.
You will have some rust remnants in your paint and it will not mix exactly even. If the room you are painting is going to be of a lighter color or white there is a very very good chance that the rust coloring will bleed through. (I don't think this will happen - but if there is enough rust bleeding through you might have to go over the whole area in something like Killz to really cover it - this would turn a quick paint job into a mini-disaster)
Verdict: Unless I was doing something like an interior of a shed I would just throw it away. I hate throwing stuff away but I also hate having to redo something and pay for materials to redo something (brushes/rollers/pans). We are talking about ~$10 worth of paint and you will be using ~$10 in materials and your time is worth?