Although it was just a single palm strike, Ye Shikai had already distinctly felt an anomalous movement of energy within Poseidon. Why would this energy transfer to him? During a lull in the fighting, Ye Shikai suddenly recalled the last time at the auction, when the chaos within his energy began merely at the touch of the stone, nearly triggering his Demon Addiction. If it hadn't been for Ji Qian's sudden appearance at that time, Ye Shikai would have lost control.
Poseidon seemed to have noticed this as well, and began to wonder. However, due to his cautious approach, he started to avoid close combat with Ye Shikai, especially since he was fighting two against one. Even with the power of the trident and the mysterious stone, he was at no advantage.
This action by Poseidon greatly boosted Ye Shikai's confidence; he had found Poseidon's Achilles' heel which was potentially lethal.