Beginner Gardening: Help! Is my cacti ok?, 2 by NoobNewbie

NoobNewbie wrote:
I bought a baby cactus 2 weeks ago and I got the plant out of the nursery soil and replaced it with a mixture of garden soil, gravel and sandy soil in the exact same pot. I leave it outside most of the time but it dosent seem too happy/ healthy now. I noticed some discoloration and what appears to be white patches(?) forming near the base( zoom in for closer look)... Can anyone tell what\'s wrong with my cactus and what can I do about it?

Thank you for your time!