I'm thinking maybe a gradient wipe combined with the displacement map and some experimentation with blend modes. Another option would be a particle system like particular. It all depends on what type of moss and how realistic do you want it to look.
If you have some sample images maybe we could give you a better formula. I do have one other question for you, how experienced are you?
I'm a beginner! It's a very simple animation so it wouldn't have to look overly realistic. I might try that and see what happens. My after effects is running really slow now, so it's making it hard for me to make improvements on my animation. It wasn't like this at all when I was making the animation, and I have a brand new iMac so I don't know whats going on. Great program but very frustrating!
My favorite tool for this sort of task is Time Displace. It's a great way to get organic effects like this. Here's my recommendation:
•Create a "root network" plate, in which the roots grow from non-existant to rather thick. You could do this with a series of mask paths, animating the mask expansion, or you could paint the roots in photoshop and animate Simple Choker. Note that the roots don't have to grow up, or in any direction, the whole screen should go from thin roots to thick roots evenly. We'll do the growing up in the a couple steps.
•Create a "leaves" plate. Since it's moss, these won't really be leaves, just tiny little dots of green, or clumps. I'd recommend using a particle system for this, but simply duplicating a growing "patch" a bunch of times could work as well. Again, no need to stagger the bits, or have it grow in any direction, just get it growing from nothing, to full bloom across the whole plate..
Precomp each of these plates, and apply Time Displace to the precomp, using a gradient layer to define the direction of growth. Staggering the growth animation between the two plates can create a realistic effect where the roots are leading the green slightly.
A simple way would be cut out your gravestone and precompose.
Make your texture in photoshop, and import the file into After Effects.
Make a mask on the texture, and make points on the mask.
Keyframe the mask so that the texture grows. This is more interesting than a
gradient because you can make interesting shapes.
Duplicate your gravestone comp.
Matte the growing texture to the top gravestone. This way you can give it
shadow or whatever.
Maybe this will give you some ideas. It's just a shape layer Fractal Noise and Colorama with some blend modes.
Tombstone(CS6).aep (Note: your browser may add a .txt extension to the file. Just remove it and you can open it in AE)
Rick- that is a terrific pattern. If I were still in mograph, it would be copied instantly.
(I got out a few years back to do the fine arts thing.)