Your cactus is reacting to being outdoors. Probably too much sunlight all at once, inconsistent watering and temperatures. Sunlight, even on cloudy days is much brighter than anything we can mimic indoors.

When moving your indoor houseplants to the outdoors it is important to acclimate them gradually. Set them out for a couple hours a day to begin with and work up to full time. Do not fertilize it during the adjustment period. Make sure it has good drainage. When it starts to green again you can give it a diluted fertilizer formulated for cacti.

https://www.thespruce.com/reasons-your-cactus-is-turning-yellow-8414643