Hello @nikv! Thank you for your comment! You already know I'm no expert on succulents, so any advice is great. I looked carefully to see if there was anything on the cactus, and with a magnifying glass, and I couldn't see anything, but you're right, its colour isn't the brightest, and the top layer is cracked near the ground. I'll go tomorrow to see if I can find something at the agricultural pharmacy. I will ask there to give me advice on what is best..never used something like that. Thanks again.
I will take photos of my other succulents because don't know many people here who know much about succulents.