Hello kind people, I have no experience with plants and bought a few cacti and succulents from Home Depot. While potting them I noticed one set of cacti have brown spots. The flesh is dried over the spots and the prickles are falling off in that area. If I peel off the outer flesh it's dark brown but fairly dry and crumbly inside the crater (see images). Is this from over watering? Is there anything I can do to save them? I've removed them from the little plastic pot they came in and am letting they dry on a plate. Thanks in advance for any advice!