Many people say this is a half-moon eye or whale eye, and it is, yet there is probably not enough context to prove what it means in this photo.
Like all dog body language, one signal is not always enough to determine a dog's real emotions. This is why many dog body language signals are called meta-signals, which you can guess by their name: signals about signals. Multiple signals create a total that might be different from some of the individual signals being made.
Imagine playing with a dog, tugging on a toy, and having them growl and show whale eye. Someone might say, "That dog has half a moon or a whale eye! They are upset!" Meanwhile, the dog's body is loose, sneezing, its elbows are bent, and its tail lazily wags. When you stop tugging, they take your cue and stop tugging as well.
Assign a value to their mood, where 0 is neutral, a negative number is a negative emotional expression like growling, whale eye or lip licking and a positive number is associated with positive body language. Let's add that up: whale eye -1, growl -1, loose body +1, sneezing +1, elbows bent +1, tail wags +1, following along +1, tugging +1. That's -2+6=4. The dog is very happily playing. Soon, you will learn that your dog's growling in those situations is a play growl; it's a very good thing to hear (never discourage any growl). You will also know that sequences of behaviours take into account this equation. For example, the dogs reversing roles after a minute is like a +5 in friendly interaction.
Nobody does body language math exactly like that in their head, but it's a good example of the multifaceted thinking that goes into interpreting how your dog feels. But people pretending they know everything about what a dog is doing in a static photo vs. a video showing the entire interaction over time just don't have enough information. It's great that people's hearts bleed for these animals, but it's a commentary on incomplete information.
My dog makes whale eye before I throw the ball, jumps onto his bed, and makes the same expression before blasting off the bed in glee. I have no doubt the dog is voluntarily playing fetch and having a great time.
