That means it has gotten established. You could cut them back a bit if you want a more rounded tree form.

I think they are just water sprouts, shouldn't be too much of an issue unless you want it to look tidier. I would leave them, the rest of the tree will follow soon enough. I am jealous though as I would love an in ground Yuzu! Yours looks like its off to a good start have you gotten any fruit? If you do end up chopping it back try and root the cuttings. If you won't use them i'm sure you could share with friends/family/fellow citrus hobbyists etc. as long as you/they aren't in quarantine zones.