If it's just Safari where you see the mask, might not be the graphics card.
From your Safari menu bar click Safari > Preferences > Extensions. If you have any installed, turn that OFF, quit then relaunch Safari to test.
If it's not an extension issue, select the Security tab in Safari > Prefereces. Deselect: Enable plug ins
Quit then relaunch Safari to test. If the mask disappeared, you need to troubleshoot Safari add ons > Safari: Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues
And check Safari > Preferences > Advanced. Where you see: Style sheet
Click the pop up menu. Select: None Selected
If it's not a plugin or style sheet issue, then run the Apple Hardware Test
edited by: cs