On the one hand, negative matter is almost certainly an impossibility. But if not, its mass should cancel out equal quantities of positive mass it comes into contact with. But since quantum field theory only allows certain particles to exist and those only as a whole (you can’t take an arbitrary bite out of a hadron like you could an apple), all transitions would have to be discreet, not continuous. If the negative mass of the negative matter could intercept the positive mass of the plate’s subatomic particles, the hadrons in the atomic nucleus should in essence fall apart and reform instantly (or rather before it’s possible in principle to observe the deconfined quarks). This process occurs all the time during natural and artificial nuclear reactions. But the products wouldn’t be a clear pathway like a nuclear reaction. It would more closely resemble the shower of new particles produced in the LHC, but on a scale orders of magnitude larger.
But as I said, negative matter is probably impossible and, although there are examples of negative mass in nature, it’s unlikely they’re ever in a form that can cancel out solid matter. Would make a hell of a YouTube video though 
