Crank arm loose (squared taper). Can it be fixed?
This question about loose crank arms has been asked and answered several times before on this site, but yours is the worst I've ever seen I think.
Unfortunately your crank arm is ruined. If the alloy crank gets loose and is ridden on the hard steel spindle wallows out the relatively soft alloy. It does not matter how tight the retaining bolt is done up the deformed crank arm will continue to come loose.
Replacement crank arm (or both crank arms) is the only solution. The bottom bracket and spindle are probably fine (unless the bearings are worn out).