Hold down GSP mat to rock with rubber band and it will stick to rock after a few weeks. Use turkey blaster to keep algae and detritus from smothering the polyps.
Welcome to reefing, you have a simple nice setup. Many reef hobbyists start out the same approach. HOB is fine, ok to have no skimmer, simple lighting is ok as long as LS in tank doesn't reach overload condition. And the types of corals and fishes are easy ones. The problem is once we are successful with easy LS we want to move on to more challenging ones. But that's the thrill of reefing. There are some parameters that can't be compromised like water. Topping up with DI water rather than tap water because overtime the accumulation of undesirable minerals will have adverse effects on LS especially corals.