I just bought, today, a newly grafted moon cactus from Walmart. I wanna know if it's okay that I get rid of the soil it came with & give it brand new miracle grow potting soil especially made for cactus?
This can be fine. Go ahead and repot it in that mix. Here is an article for more information on their care: https://www.gardeningknowhow.com/ornamental/cacti-succulents/moon-cactus/care-of-moon-cactus.htm