I don't know mulberry plants very well at all (we have wild mulberry trees here, but I've never seen them potted), but in general those small brown holes in the second pic look like a fungal infection :-O

Were I in your shoes, I would isolate the plants from any others immediately to prevent a spread. Even if I'm wrong about the fungus, isolating it can't hurt. Put it somewhere with low humidity and very good air flow.

Step 2 would usually be to remove any of the leaves that are affected. It looks pretty widespread on your plant, though, so I'd wait for someone else to chime in before doing that.

Step 3 is to treat it with fungicide. There are a lot of kinds that you can buy, but a simple solution of 1/2 tsp of baking soda to 1 gallon of water might be enough; just spray it down with the solution pretty well.