You see it more often on bigger color crts as the tiny black and white screens in the macs (or CBM PETS or the Vectrex) only have rather tiny amounts of anode voltage. More in the range of 5KV then the 25KV or even more on 29inch Color tubes.
IF it really bothers you, then you can discharge the tube, disconnect the anode cap, clean around with isopropanol and reconnect everything, with or without dielectric grease. . The red big circle on the screen is more paint like and between it and the rubber cap there is the grease sealant.
Personally i even wash the screens in water, which is totally okay unless the dag (the black paint on the tube) is water soluble (it is on the vectrex, ask me how i know) but the dag can be repaired with graphite spray paint easily.
PS here you can see the grease on different systems:
Now on to the monitor. Standard stuff here. Geometry problems and poor convergence around the edges making things blurry: Cleared out the broken bits from the deflection yoke: (This step is mandatory, I learned. I initially tried to skip it and immediately regretted it as I heard...
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