Village houses sometimes generate with mossy cobblestone in place of regular cobblestone, this never happens with any of the variants of cobblestone (cobblestone stairs don't appear on villages as mossy cobblestone stairs, cobblestone slabs don't appear on villages as mossy cobblestone slabs and cobblestone walls don't appear on villages as mossy cobblestone walls, only regular cobblestone does this.

Here are some screenshots from an abandoned village, this happens on regular villages as well but it is less noticeable due to the mossy blocks being less common, so i used an an abandoned one due to this behavior being more common here:

Structure with most amount of cobblestone stairs to confirm this on regular villages: