Is negative mass antimatter?
No. There is really no such thing as negative mass. Even antimatter has mass, which is always a positive (that is, greater-than-zero) quantity.
There is, though, the method of effective mass or apparent mass, which can be negative. When an object is submerged in water, its apparent weight is decreased by an amount equivalent to the weight of water it displaces. For instance, if a 10-pound rock is dropped into a bucket of water and displaces one pound of water, the effective weight of the rock under water is nine pounds.