I would have posted good useable photos so we can actually see what is going on? In the end though it is a bike and a folding bike at that and unless it is something super high end and not utilitarian, I wouldn't worry about it. If I am riding it often keeping it to a base level clean is fine but embedded stuff in the paint or whatever it might be isn't a worry it won't effect the ride and won't cause any safety or shifting or braking issues so I won't worry. For me a folding bike would be a bike I wouldn't mind getting banged around a little certainly not to an abusive level but enough where the paint not being perfect is not a big deal.

If I were really truly worried, alcohol, Rock N Roll Miracle Red and some Dr. Bronner's soap mixed together and sprayed on and wiped off well generally will take care most stuff and is not harmful (aside from to brakes and braking surfaces but anything you spray on them that isn't alcohol or a specific bicycle brake cleaner is bad.