Issue:

Blue stripes suddenly appeared in my glyphs and I cannot get rid of them.

Solution:

These lines are so called “Alignment Zones.” They are automatically generated in hinted fonts, such as the fonts output when exporting OTF or TTF from TypeTool, but you cannot edit them manually in TypeTool. However, you can choose whether or not to display them. To show/hide the Alignment Zones, press Cmd-Shift-Z (Ctrl-Shift-Z on Windows).

You can control the visibility of other layers with items on the View > Show Layers menu.