I know this is a bit late but you have dog vomit fungus. Getting rid of the rest of the stump will get rid of the food the fungus likes to eat (decomposing wood) and thereby get rid of the fungus... in theory at least. ;^) Hope your lawn recovered.

Here is a link that might be useful: Dog Vomit Fungus

Actually, thanks but it's actually definitely NOT dog vomit fungus (best name for a fungus ever?!). I've had that in other parts of my yard and my mom has it randomly in her mulch a lot (the spore stage is rather off-putting!), but it's not what we have. Ours looks much more like a mushroom, never had the yellowish stage, and doesn't really go through stages at all, actually - it just is what it is until the lawnmower chops it up.

The spot is getting better this year, though. We again tried to fill it in with grass, but I doubt it will make it all summer (a lot of it is already dying off). There's really no way for us to get rid of the rest of the stump (it's very large), so we'll just deal with it!

Thanks!