You can currently craft marble slabs by placing 3 marble brick in a vanilla crafting table like any other slab but the recipe does not show up in jei.

Also I find it weird that you need a luminous crafting table and regular marble to make marble stairs but a vanilla crafting table and marble brick to make slabs. Seems inconsistent to me.

AS has no slab recipe, its why the slabs behave oddly, they are creative only.

Some other mod has added those recipes.