As RobMcC said, this magnet is not toxic, when people describe losses due to magnets, this is due to rare earth magnets which are typically copper and nickel plated and it is the copper and nickel which cause losses and their is some possibility the neodymium itself is toxic as it is usually an amalgam with boron and this and the neodymium could have toxic effects but not on the level of copper and nickel. We do not use such magnets in our magnet holders, while we use rare earth magnets, they are non plated, raw magnets and the magnet holder you show is a much older design, this design is a ferrite magnet, it is a manmade ceramic, what most impellers are made out of and is entirely inert. Under the magnets is a plain iron plate, it is not galvanized or plated in any way. While it is possible the iron plate under it is rusting, this tends to have a distinct appearance, usually a bullseye like stain where their is a hole and the rust emanating from the hole forms a ring like stain. When the rust is flaky like this, the main culprit tends to be attracted iron particles, while their are numerous sources from sand, household particles from sharpening knives, using steel wool, driving in nails and screws, left over from the machining processes at the factory, the most common source is just GFO dust, GFO is literally just pelletized rust which should give you a feel for the toxicity in this case, at worst if the magnet has a rusted iron plate, you have just made GFO, nothing more. My hunch is, that if you soaked the magnet in a mild acid and cleaned it well with a brush, you will remove the staining and it is not the magnet holder that is rusting, it simply picked up material that is rusting. The current model magnet holder is neodymium and is fully coated, the only real benefit is that being glass smooth, very little can stick to it, compared to the 3D surface on the older design.
