It looks sunburned. As I’m sure you’re aware these are tropical forest cactus, since you cross posted this in the tropical fruits forum. So these guys aren’t huge fans of unexpected hot sunny days. I’m sure once acclimated it’ll be fine, but this process should be done slowly, and if you live in a super hot climate you may want to put in morning sun/afternoon shade during the warm months.

Thanks, Kara. I see that you are from the Bay Area. I live in a very mile climate just south of San Francisco where you wouldn't expect the heat we've had this year. I kept the plants out of direct sun until they showed an inch or more of growth, then eased them in. I did not expect sunburn even with the temps we've had recently, but I guess it hadn't yet acclimated.