Yes, plastic coated magnets are safe for aquarium and reef use.
Plastic coated magnets are commonly used in both fresh and salt water
aquariums, to hold up coral, pumps, frag rocks, for cleaning and more.
Our
Aquarium Magnets
article talks about this in further detail and gives suggestions based
on the glass thicknesses. We do not recommend using rubber coated
magnets in aquariums.