Judging from the flaking, that CRT looks worn out. If you can find a nice one, replace it before good ones become even harder to find.
On these classic Macs, these problems usually come from a faulty analog board. Typically it’s bad capacitors or a flyback transformer. If it’s bad caps, this can be fixed but you may need to substitute some of the obsolete values and tantalum caps. If the flyback is bad, then you might as well get a tested/refurbished analog board and use this one for extra parts unless you can do the flyback swap.
CAUTION: CRT’s can (and do) KILL. Do not wear any metal jewelry when you have the CRT and analog board exposed and discharge it.
Most CRT’s made from the 80’s/90’s-present have a bleeder resistor, but this should be assumed to be burned out and discharged to be sure.
If you don’t know how to work on a CRT safely, read up on CRT safety before doing the repair and opening it. Take a look at this answer for CRT safety but DO NOT rely on it as your sole information source: Disassembling an old Apple monitor