Milts will get these marks, I hate them too, seems to be normal from times to time. It's when they spread from leaf to leaf then it becomes an issue.
Don't allow milts to dry out completely, they thrive in high humidity. Also ... easy on the ferts and good air circulation is important to milts. If they continue to spread out, then a fungicide should probably be used.
Take , a wet tissue and run over the under side of the leaf if orange ish smears show up then it is mites , Safers soap from Lowes or any of the big stores will kill them , spray every 3 days for 2 or 3 weeks , rinse the leaves in water before using each time , let dry out of the sun .
Having thin leaves Milts. are prone to water damage , water sitting on the leaves that might of caused the spots , You can use straight peroxcide on the spots and then a little cinnamon to keep them dry , if they continue to spread and are soft then the leaf can be removed .. Hope this helps .. Gin