The force said it had arrested a 19-year-old woman in Sha Tin on suspicion of conspiring to commit arson.
Meanwhile, a 24-year-old man arrested earlier appeared before Eastern Magistracy charged with arson.
The court was told the man had been attacked and threatened by officers following his arrest.
The hearing was adjourned until September 27 and the defendant was remanded in custody.
The alleged incident happened at around 2am on July 1, police said earlier in the week. Firefighters put out a fire on the slope. No one was hurt.