Hmmm. That’s interesting. I suppose you could attack it from several angles. I mean, you could create curve splines and animate them moving overtop your surface as the paintbrush element. Then on top of that, have dynamic paint using the “spread” option so that it exaggerates the effect. The only issue with Dynamic Paint is resolution. It has to be UV unwrapped and things can get pretty slow after around 2048 pixels square (the size for baking your Dynamic Paint onto your canvas).
Otherwise, you could use masks in 2D to animate over the landscape, transitioning between the white version and the dark version. You could use the projection mapping modifier and project from the top view an animation of the tentacle masks slowly moving outwards.
Hope this helps!