About terrain "decay"

[Gallade]

In my village, we cut down a large area of forest and let it sit for a couple weeks so that it would turn into grassland and thus be easier to pave.

But in the first area of the village, the one we used "intensively" (for growing crops and breeding cattle), more and more tiles of grasslands are turning into what I think is "moor" (The lighter green terrain with thick weeds).

Is there any way to slow down this process, and does it come with a risk?

[Arcanist]

it comes with no risk. if you want it gone just plant grass, or plow it again and wait for it to become beautiful pink heath.
If you do want it gone, get rid of all of it, as it spreads. Simply plowing it won't change the tile type.
Iirc it doesn't spread over pavement, but I could be wrong

[MagicManICT]

No, it won't spread across pavement, though if you have a break in any of the eight tiles around a particular tile, it can spread that way. Personally, I prefer heath as you can get blueberries and a couple of other forageables on it you don't find on other grasslands.