Not surprising as both BBA and staghorn are related actually, both of the Rhodophytes kingdom. There could be some BBA on that rock, actually. I was looking mainly at the leaves.
Typically, staghorn is found in more alkaline waters. 9/10 when you raise your co2, whether you use excel or other methods DIY, pressurized, it will go away. If not then. Lights lowered means less co2, so that can help as well. However, there is always a possibility your tank isn't cycled or going through a mini spike as it likes ammonia. Have you checked this recently?
Without Algae, death of mankind would be inevitable.