Ceramic magnets contain powdered iron oxide to enable the ceramic magnet to be magnetic. Iron is not considered a harmful heavy metal and should do no harm in small amounts and isn't likely to break free from the iron /ceramic compound.
Iron is also taken up quickly in reef tanks by algae and other organisms.
Other common magnets contain aliminum, nickel ,cobalt and iron . Nickel is toxic ;aluminum can be at certain levels. Not sure about cobalt. Some other elements are present in trace amounts, perhaps copper but I don't think it magnetize well enough to be included.
In genral the trhreat is from nickel and not copper. Many have had coated magnets leak out and harm corals and other invertebrates.