From the rescue:

“Our joeys stay very well-hidden most of the time, so we don't get a lot of great photo opportunities. But, today, had to clean out these joeys' favorite hide box, so they came into the open and "smiled" about it. That characteristic grimace is actually not a smile at all, but an indication that the animal is stressed and scared. It means they want me to go away, and that's a good thing! Keeping wild animals wild is the most important part of our work.”