The shower screen bolt does tend to over-torque itself. If your model Silvia has a hexagonal head bolt, then its best to remove it with a socket wrench.
If its a slotted round-head, get a stubby screwdriver with a blade which isnt too loose a fit, press it firmly into the bolt to prevent the blade slipping off and damaging the adjoining screen as you try to loosen it.
Persevere, but be careful.
When you have removed it and scrubbed the screen -- therell be a lot of black gunk in there---replace the bolt finger tight only. Thats all it needs.