The picture isn't so obvious.....the plastic around the magnet has split. This is rust.
One of my MP60's did the same thing. It looked like the magnet just expanded or "popped". The plastic around it was all split and the magnet itself was swollen. Not sure how to explain it but it looked like when you get particle board or sheetrock wet. Never seen a magnet do that. The sent me a free wet side pretty quickly. Just call them. I'll add though that mine didn't rust. I clean them weekly and noticed it wasn't spinning so I must have caught it right when it happened or shortly after.