My algae free was covered in coralline algae. I actually stopped using it and let my clowns lay eggs on it for almost a year. Finally i decide to clean it and all it took was about an hour in citric acid from Amazon (milliard brand).
Not a fan of leaving magnets in the tank, not so much because the magic of magnetic fields or anything, but they're usually going to be a magnet that you normally would not want in your tank that has in some way been sealed. I'd rather not have to test out how long that seal will hold underwater as all sorts of gunk and crap grows into any little crack or crease of it.