If that was the first time you changed the primary fluid, I wouldn't worry too much about a relatively large clump of debris on the magnet. If it looks that bad on the second change, then I'd say something is wearing more quickly than normal.
Don't forget when looking at the magnetic drain plugs for the primary and trans that those do not have any other form of filtration to remove particles, like the engine does. I always expect to find more on the primary plug, less on the trans plug, and the least on the engine plug. Since changing the primary design in 2007, the compensators do create a lot more debris. Add to that the clutch material that wears off and it can look nasty. Many have gone back to changing the primary every 5000 miles with the engine oil because of this.
Jerry