You could consider doing a freshwater dip to check for external parasites like flukes.
There are three basic types of flukes that generally attach in different areas. One type usually goes for the skin, another goes for the gills and the last one goes for the eyes. Could be one or all of them bothering your MB. The FW dip works best for the skin flukes and they readily spasm and fall off. The ones in the gills will also come off but some will be protected by the mucous within the gills. The ones in the eyes sometimes react, but usually require a QT treatment of longer duration with a praziquantel product like Prazipro or General Cure. And sometimes they are tough to remove and hypo works pretty good, and is a chemical free treatment. It just takes time. It's very difficult to see flukes in the small eyes on this size of fish.