Shimano's external BBs are not easily serviceable by the end user as per their design intent.
My attempts at removing the dust cap without damage have been unsuccessful. But its no big deal since the bearings in the BB were already toast (rusted in place). Even if the dust cap was removed its difficult to pop out the bearing without an expensive bearing puller tool i.e. Enduro tool because there is no offset between the bearing and the inside of the cup.
I ended up replacing the entire BB with Hope which is much more user friendly as seen in their maintenance video i.e. easy to remove the seals to regrease and pop out without having to use an expensive bearing puller.
Near the middle of the video they show how easy it is to remove the seal & pop out the bearing.
Get a thin flathead screwdriver in there and gently pry around the edges. It will start to pop off.
The Shimano external BBs really arent that hard to service, you just have to do it frequently. My shimano BB ran dry and felt really rough so I bought an Enduro bottom bracket and it's much much better.
hit the whole thing with a hair dryer for 4-5 minutes, on disassembly/reassembly. carefully pry out with a flat-bladed screwdriver, working your way around slowly. pop the seal beneath. degrease/grease just like a ck headset (wd-40/simple green/heat/air/grease of choice). hitting the cartridge bearing with heat just prior to greasing with a fine tip grease gun tends to let the lubrication agent move around more easily.