Winter Mice

That plaster looks old so you need to go easy on the cleaning or you will ruin it. Do not pressure wash it.

I would first try SLAM level chlorine for a while.

If that does not work I would try hand sanding an area of the plaster with 80/100 grit wet/dry sandpaper or polishing pads like described in Plaster Installation and Maintenance - Further Reading
 
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