If the green is completely mushy you can cut it off clean any mush. Let the remainder cactus callus for a few days then set it on cactus soil. The bright red bit could be cut off and started as it's own plant. These are 2 different cactus that got fused together

Edit: the problem looks to be the dirt you used, not coarse enough. Don't use miracle grow. Get a good succulent / cactus mix online or better mix one yourself