Both Mamoru's whole-hearted selflessness and true powers came full-circle in the Dream arc. As the Dead Moon closes in on the four connected souls - Earth, Elysion, Mamoru, and Helios - the destruction begins to affect Mamoru's body and health.
He wakes up one morning, coughing up small spats of black blood into his palm. The spirits of the shitennou watch over him, and realize that the aura around his body is weakening. They want to help their prince, but knowing their powers in their current state cannot do much, they simply watch and worry.
Although he is quickly weakening, he wishes to keep his plight a secret from Usagi. He doesn't want her to worry over him while the battle with the Dead Moon intensifies. Mamoru would rather keep his worries and troubles to himself, because he would otherwise feel like a burden to his beloved.
When Usagi does inevitably discover what he's been hiding, he immediately tells her, "Stay away! This curse...you might catch it!" He winces, and thinks to himself, 'I can't even help you with this body! It's truly admirable that despite his pain and possible impending death, he only seeks power to be able to help and protect Usagi. His insecurity in his own abilities is exposed once again, as is his extreme will to protect his loved ones.
Mamoru's powers are still dormant at this time, but even if he never had any at all, Usagi wouldn't care. She would always be willing to stay by his side, and fight with him to defend their future together.