Why does my dog eat gr ?.?

I have a older dog that likes to munch on gr and weeds , I don't know why . She is about 5-8 yrs old and she eats well ,but she still likes to eat alittle gr or weeds on occasion . She also has runny eyes all day , 7 days a week . She also has cataracs ? in both eyes . Is there anything I can buy to dry up her eyes , they are blood shot most times too. Any suggestions ?.