Can I take Mucinex (guaifenesin) if I have glaucoma?

No decongestants. Avoid mucinex-d if you have glaucoma, but standard Mucinex (guaifenesin) should be fine.
Maybe. As long as you don't have untreated high risk narrow angle glaucoma, then other the counter meds warning about glaucoma should be ok. Mucinex (guaifenesin) can affect pupil dilation (especially if combined with a decongestant), and thus potentially trigger acute angle closure glaucoma, an eye emergency (reason for the warning).