I recently planted a few seeds gathered from a Tsuga canadensis cone in soil gathered from a garden full of weeds. I can't tell if what I have growing is a hemlock seedling or just another weed. See photo:
This is not a Hemlock seedling but a young specimen of the family Amaranthaceae, almost certainly an Atriplex, known in English as Saltbush or Orache.
You can tell by the shape of the opposite leaves and the 'frosty' look (not sure if frosty is the correct English word)
This specimen looks quite similar to yours. Taken from here
As North America is very rich in species, I don't think anyone can be more exact than Atriplex.
Atriplex grows in salty soils such as deserts and seashores, but also in moist environments. Most species prefer bare soil, so are also well adapted to grow between crops, so yes, this might be called a weed.
No, this is not a Tsuga canadensis seedling.
A hemlock seedling should look something like this: