Cool tank. Not many people keeping seagrass! I've tried Halophila a few times but never had much luck with it. My flameback angels seem to eat it before it gets going.
A few weeks ago I found some Thalassodendron ciliatum growing on piece of coconut husk that was half buried in sand. Thalassodendron is different to other seagrasses in that it doesn't need a deep substrate. In Mozambique I've seen it growing on almost bare rock. It's been in my tank for 3 weeks now. Some of the original leaves are dying off but there are 6-7 new sprouts that are growing about 5mm per day. I'm not sure if this species is available in the US (seems to only be common in East Africa) but it could be a good one to try!
Not my pic but gives a good idea what it looks like:
