Is she getting sick from it, or having loose stools? If it's not affecting her badly I wouldn't worry too much about it. One of my dogs (one of 7) is a constant grazer when we're outside. I've never seen anything like it. We have a small farm and the dogs go out with me to do chores. When we're walking through the fields he heads for a patch of tall clover and grazes at the grass that is taller than the clover. The way he tears if off looks like a goat or sheep grazing. When the chicken coop door gets opened he also rushes in there to grab any eggs that are on the floor. Eggs and grass, nice diet.
Of course, he
is on a diet and thinks I'm starving him, so maybe that's why he's started grazing
It doesn't make him sick or affect his stools so I figure I can allow him his little eccentricity.
If you want to make her stop, you could try taking a favourite toy when you go outside and distract her with that when she shows interest in the grass clumps. When she switches her focus from the grass to the toy praise her and initiate play. Clicker training might help too. When she shows interest in the grass, call her name and if she looks to you click and treat. As she gets better at that you could start asking for some other behaviour (like sit, lie down etc.) Many clicker trained dogs seem to keep their focus totally on their owner/handler. Of course, if you're taking her out to potty you don't want it to turn into a play/training session, you want her to get her business done quickly so if you do that on leash and keep her in an area that's clear of the clumps that should help.
As for clay, sometimes animals will eat that if they've gotten into something they shouldn't have. They seem to instinctively know that clay will help to absorb stuff in their gut. I remember seeing something on animal planet where a tazmanian devil at a zoo tried to snack on a cane toad (deadly poisonous) but realized it's mistake and began gorging on clay. The zoo vet had to operate but the tazzie devil did survive. I've heard of similar behaviour in other cases too.