I'm not familiar with the seedling you show.  Most identifications require some of the plant structure and usually flowers unless a person just happens to be familiar with that plant or weed.. When you get something larger you can take it too the Center for Urban Horticulture which has an herbarium (a museum of pressed plants) and plant ID trained students.  Or you can take to the Master Gardener Plant Diagnostic Clinic at CUH.  CUH is located on Mary Gates Drive that turns off of 45th St. down by the UW stadium.

I've included a photo of a tangerine seedling so you know your seedling isn't a tangerine.  According to a website tangerine seeds should be bought but rarely flower or produce fruit here.