Well, you could post a photo and we can see what you are talking about; or you could call Monrovia and ask them what rootstock they sell to HD. Sometimes those vigorous "suckers" are just watersprouts.

Forgive my ignorance, I just looked up water sprout and I think that must be it, as it originated below the graft line but above the soil.
Not much point keeping it now, I think, beyond the demonstrated toughness. It seems unlikely to do well for fruit in terms of quality or quantity.

Definitely rootstock if it's from below the graft. If there is no cultivar above the graft, better to just replace the tree. You can always try to draft, but then, you'd need to know how to graft , and have access to clean budwood. Not hard to learn, but it's a bit of a process. For most folks, simply easier to replant.

Patty S.

The rootstock is likely a trifoliate orange.

Thanks rugbyhukr, you are spot on I think, having just googled that. Seems that my new tree would not bear fruit for many more years, and then they are pretty awful tasting fruit anyway. I'll probably go guerrilla-plant it somewhere I may one day see it fruit, but not in my postage-stamp of a yard.
Thanks everybody!