I am no authority on this type of material... you are far better off with anyone other than myself, I'm more of a deciduous guy... and, contrary to most people here, I am reluctant to do much with fertilizer(s). The moss I'd personally pass on...
If you could pry the overlying rock off the top of the root system, to take a look to see what you can develop, then replace the rock with smaller pieces like a jig saw puzzle, you could monitor the root progression over time. But thats just me... I have a lot of time, and no one to pester me once I have a plan in mind.