This is a seedling from a Shasta Gold mandarin, a triploid variety, which rarely ever has any seeds.
At first I thought something else was growing in there, perhaps a weed, but then I saw on closer inspection that it was citrus. I had planted two germinating seeds in there.
These are both seedlings from Shasta Gold but one has unusually elongated leaves. Perhaps it is aneuploid.
The other has very thick stubby leaves and reminds me of what tetraploid citrus might look like.
Funny things can happen when you grow citrus seedlings from triploid cultivars, and I don't think Shasta Gold produces nucellar seeds.
All of the Shasta Gold seedlings I've tried growing (let's just say I went through a lot of fruits to get those seeds) have turned out weak and slow growing, oftentimes kind of stunted, unusually so. I don't think that's a coincidence.