well joncon said the bird feeder has sunflower seed hearts, so there probably aren't hulls. But even with the already hulled hearts, there will be a lot of dropped pieces and other trash. And it probably is mold.
I stand by what I said that even if you clean it out, put down new dirt and new seed, right under a bird feeder is not a good place for growing grass. You have two choices... move the bird feeder to a different location every month or so, so the grass under it doesn't get too killed (in which case you could do the new dirt, new seed routine and let that spot grow back) or leave it in one spot and give up on the grass in that spot, just mulch it.