Hm never tried grass yet, lol, would think grass is for sheep and cows

But might be worth a try if you can be sure whatever you bring from outside isn't contaminated with herbizides, pestizides or fertilizer. Dandelion leaves I meant to try somewhen but forgot.
For feeding veggies, you could as well make your own fish food by blending peas, spinach, bit of paprika and whatever together, add some gelatine with a little bit of water, put that on a tray in a layer and freeze. You can then break off pieces when you need it, that stuff sinks (but can make a mess as well in the tank, especially if you used too much water). It should be thawed first though.
I've no clue about African cichlids but some of them should really have a vegetable based food mainly - with too much of protein as in meaty food they can get bloat and die. Is only for some species though I think, no real idea.