You'd need to tell us exactly what terrain block you used. There are blocks recommended for POI building so that no matter what biome it spawns in it uses the local block. This way it doesn't look silly, but there are specific blocks you can use also that are just that specific terrain.
As for placing terrain ontop of your build that would allow zombies to spawn since it is not a player-generated block. Land Claim and bedroll rules will apply also.
You're better to not put terrain on player blocks. In A17 I did this with tilled soil (just plant and it grows) and no zombies spawned on it, but now that you are using farming containers this probably doesn't work anymore, but you could try it.