I have owned a Mammillaria Nejapensis for the last few months which began showing unhealthy yellow spots a few weeks back and seems to be getting progressively worse. A photo of the cactus is attached.
Unfortunately, my room barely gets any natural light, which is why I chose to illuminate the cactus with an artificial plant-growth LED array from Amazon. I use a timed outlet to expose the cactus to 8 hours of LED light daily.
Would anyone happen to know if it is likely the the yellow spots are a result of insufficient lighting or some other cause I am unaware of? If it is the former, is there a lamp that might do a better job than the LED array I currently have?