The rootstock is different and that has started growing. It could be anything, lemon, sour orange, trifoliate orange.. Someone else will make a better guess.

It is likely that you will eventually loose the scion (kumquat) if you don't remove the rootstock sprout. The sprout will vigorously outgrow the kumquat and the graft will be likely be dropped. If it were me, I'd prune off the sprout ASAP.

we would need a pic of the trunk.. where the two different plants attach.. to explain any further as to how to proceed...

good job protecting your plant.. from the idiot with the weedwhip .. lol ... i spotted the utilities back there.. and said to myself.. that would quickly become a garden bed.. so i wouldnt need the weedwhip ...

ken

Lots of flowering cherries and old rhododendrons (these used to be grafted instead of grown from cuttings) around here where people have never cut off the rootstock sprouts. Grafted plants often have to be watched for this if you aren't going to end up with most of the top coming from the roots instead of the variety you wanted originally.