Unknown type? It's staghorn. (Compsopogon). Cause: Low CO2 and some NH4. Add more CO2 or use less light (hours or intensity) and always change water after working in the tank. If the tank is still new, maybe it's not cycled yet and there's some NH4 around. If so change 50% water twice per week over the next weeks.
Try to increase biomass, even it's just temporary plants that are not part of the final design.
If you do a search on Compsopogon on this site you can find more info.
It may also help your co2 distribution/stauration if you put your diffuser next to your filter to get it to flow with the current better. +2 on light reduction and OD excel for a few days until it turns red. Its red when it dies.