I think it's sunburn - but it's weird to me that cacti were in your gifted terrarium. They are not a plant I associate with the growing conditions inherent to a terrarium. So many cacti grow naturally in hot, dry climates for at least part of the time and terrariums that I've seen have plants adapted to warm, moist - and generally continually moist - conditions which would be a death sentence for many cacti. I can't identify your particular cacti, but they don't appear to be the sort which would naturally thrive on the floor of a tropical jungle - conditions found in a terrarium. Just my thoughts. If those were Gesneriads of some sort instead of cacti it would make more sense to me.
