It is absolutely not part of your plant. Seeds of something else got into the pot and germinated. It's hard to tell at this early stage what plant it is. You can try potting them up by themselves and see what comes of it.
I mention that the plant (ivy) wasn't in that pot when I bought it. I moved it to another one (the one that's in the photo). As you can see there is also a little of parsley (which I believe is dying or something). Thanks for the answers.
EDIT: Isn't it mysterious that the new leaves of the ivy that had recently grown are located so close to the weed or whatever that plant is? I'm talking about the big and greenish young ivy leaf that you see there.