This plant was doing so well and now is losing its color and has brown patches. Any idea what is going on? Thanks so much!
Succulent not doing well
It does not look good! Your plant looks like an euphorbia to me. Is the bottom of the plant all infected? Are the brown spots soft? Hard to really see anything in your small photos. Some cactus and euphorbias do grow "bark" at base of plants as they age. This is normal. Can be called corking. https://www.euphorbia-international.org/euphorbias/pests/about_euph_pests_corking.htm
But if soft, that is not normal. And you do not get brown spotting all thru out plant with corking.
Looking at it from a great distance, I would say it is a goner. If you take it out, bag all of it and sterilize the pruners you used on it. I would not replant in exact same place. And beware, if it is an euphoria the milky sap is toxic. Some euphorbia sap can cause blindness so wear glasses and gloves (can cause skin irritation too) and do not touch your face.
Off hand I would say fungus though hot Arizona would not seem to be humid enough to foster fungal diseases. But of course I am wrong! https://cals.arizona.edu/extension/ornamentalhort/plantprotect/pldiseases_urban.pdf
This article may help.
Scroll down to bottom photo and explanation