Keeping zoos on the top of the aquarium will not hurt them, even with metal halide lighting. You will have to slowly acclimate them though, moving them up a couple of inches every day.
Keep in mind that many zoos/sea mats often grow in areas of the wild where the tide leaves them exposed to the baking sun for hours, without a drop of wate on them. They can handle the top if you acclimate.
The position in the tank will also affect their morphology. If you have them at the botto, they tend to elogate their stalks and resemble flowers. If you have them neat the top of the tank with strong lighting they will grow with much shorter or absent stalks, resembling mats.
