Video mapped to item within playing video

Hi all,

I'm relatively new to FcpX, Motion etc, so upfront apologies if this is either already answered, or very simple.

I've filmed a flowing stream (circa 3 minutes)
Within the shot, but slightly moving is a large rock
I'd like to have another video playing on that rock, but with all the angles and non standard shape of the rock, not just plonked on top. I'd like to make the video really look like the existing rock in all dimensions. If possible, I'd like the ability to further transform the additional video (grow/shrink etc), if it could also motion track/position keyframe, I'd be very happy

How do I do this? Thoughts I've had

Does a plugin exist
Is is done by selecting a colour/area within the prime video and playing with the channels
Can I transform/distort a new clip for placing over the rock using the usual tools, I only recall the four handles, I'd need more than that to match the required shape
Masking?

Thoughts, help would be appreciated
Thanks
Steve