I wish you find all the best you want! To all the Duolingo users especially Chinese learners on this very day of Mid Autumn Festival.
You can put literally anything into a mooncake, from fruits to nuts, or even Chinese dishes, such as Kungpao Chicken mooncakes. The most popular ones are 五仁(five nuts), 豆沙(grounded beans with sugar, usually red bean), 莲蓉(grounded lotus seed with sugar), 黑芝麻(black sesame), and 蛋黄(duck egg yolk, marinated into a solid. I don't know why it turns solid, but this is delicious).
It's something called 咸蛋黄 (Salty egg yolk?). That's usually the middle bit of traditional mooncakes.
Yes, it is marinated duck egg yolk, which is no longer a liquid because of marination.