I can't find any pictures of my seedlings, but trust me, they were ugly. I germinated 10 seeds and they all popped. But one didn't break soil at all. Three others were so crooked, and yellow, and brown....I just tossed them. I kept 6 out of the ten seedlings because that's my legal limit here in Maine. One of those was stunted pretty bad, here's a shot of her at the beginning of week three...
Looking at your cotyledon it's looking a little off-color, but 8 out of 10 of mine were off-color. I'd give it a little more time. Also, I think one of the other posters said it, get something in the soil to thin it out (make it lighter and more able to provide air and drainage for your roots). I use perlite.
It sounded like you were planning to pull her out of the solo cup and modify the soil? Be SUPER careful with that taproot. I had to transfer one of my seedlings after the taproot had shot out about half an inch. I was successful, but I was using peat pellets, which are fluffier than what you look like you have in the cup.
My two cents worth, they call it weed for a reason. In the ten years I've been growing, I've made some serious mistakes and I've never had a plant just flat out die on me. For all the problems I had with this grow so far (this is my first attempt at growing autoflower from seed) the girls have been resilient. The above picture is a shot of the smallest of my six seedlings at the end of week three. Here's the tent today (that slow-growing seedling from above is the plant in the middle front).