I don't understand. You added more grain after the mash?
If so, that explains it. Unconverted starches would be slimy. In order to utilize the grain, you have to mash it. That's what converts the starches in the grain to sugar. If you didn't mash it, but added it after the mash and then sparged, you'd just release the starch as you wouldn't convert the starch to sugar. So you'd have a starchy slime in the grainbed/wort.