Looks like the growth is coming from below the graft. If so, not a reversion but understock growth. You can remove it at any time.

tj

I agree. And I wouldn't hesitate in doing so as soon as you see that happening. The rootstock - most likely just the species Acer palmatum - grows much more rapidly and vigorously than does the grafted scion and will easily overwhelm and smother it if not attended to promptly. It diverts energy away from the desired top growth.

Thanks so much to both of you for your advice!