That may be overstocked. Algae and water quality can be hard to control in hex tanks. Most light fixtures can't properly reach the bottom and it takes a lot of flow to oxygenate them as well.
One issue you're likely to run into with a HOB
refugium is fit. You might not have enough length to fit any but the smallest HOB
refugiums. How long is the back panel and is there already a filter on the back?
Honestly, with a 30 hex tank you'll be better off investing in a skimmer. It would go a long way to oxygenate a tall tank with a small surface. It'll also help pull out excess organics that feed the algae. I know a refugium would help with both issues, too. I'm just not sure you could hang a large enough refugium to be of help. Those glass panels are narrow.