Any rust would be the same rust we use every day as GFO, there are no toxic metals in this magnet construction, more often than not what is rusting is iron the magnet picked up, including GFO dust and not the magnet itself, the actual magnets are ceramic and cannot rust, there is a polyurethane coated iron plate under the magnets that can potentially rust if the coating is breached. That is a magnet design from about 5 years ago, the current ones are rare earth and fully encapsulated. Magnets from 3 years ago would be similar but the magnets are round on one side and matched to the contours of the casing. The main issue with these is the 3D surface gives lots of crevices for accumulated bits of iron stuck to the magnet to rust and cause staining. I would simply clean them with a brush and vinegar as they are likely not rusting and even if they were no harm can come to the tank. The current 6025.512 is smaller and fully encapsulated if you prefer to replace them.