I took mucinex (guaifenesin) last night and now today i have solid stool but it's green. should i be concerned? or is it just the medicine passing through?

No concern: it is really hard to determined the exact reason for the color of the stool if the medicine liquid or gel or tablet has a green color to it might contribute to that but in general i will not be too concern about that concentrate on the main issue your URI hydrate well eat soups and rest

If..: If you are having symptoms related to a viral or bacterial infection the changes in stool quality can be associated to that rather than Mucinex (guaifenesin). The need to follow up on your condition and the urgency to be seen depends on what other symptoms you are presenting. If just mild symptoms drink plenty of fluids and rest. If you feel very ill go to the ED for immediate evaluation.