Excess of money is not harmful if you don't allow it get over your head. The statement excess of anything is harmful is mostly used in relation to things taken into the body. E.g, food, exercise, water, alcohol, etc. Since the body has a limit of these things it can accommodate at a given moment, when such thing is given to the body in excess, it becomes difficult for the body to process and this stresses the organs of the body.
But since money isn't taken directly into the body, having money in excess cannot be harmful unless we allow it get into our heads and cause us to engage in harmful habits or practices. If you can still be yourself after having the excess money and not allow the money control you, it can never be harmful to you.