you don't want to start your HC off with that in the tank, it will spread into it like wildfire if you don't have a handle on it. I'd remove the plant or cut off the affected leaves. if you remove it you may be able to treat it in a spare tank or something.
Sorry it took so long to reply. I would remove either the leaves or the plant, because this is a pain to deal with later. Also, there have been a lot of recent posts on hair algae. I would read those, because I found them quite helpful.
I don't know how others have dealt with this, but IME mollies, esp. the smaller true black mollies, are like algae assassins, and they seem to LOVE hair & black beard algae. I added them to my 10G when it was overrun with both, and they had it clean within a few weeks.