@Merilirem
The novel explains it as he's making this sniping attack. The reason he can still use his magic.
Mana can't be seen with the naked eye, in this series. Except for the people from Shadow Garden. The reason for this is the density of the energy. Mana is like air. The mana density of the Shadow Garden girls is like mist. The main character's mana is liquid so thick nearly solid. The main character figured out a way to refine his energy into a much purer and more powerful form. It also shows up as a pure violet color, for some reason.
That's why a single drop of the main character's mana is a few hundred times stronger than the same volume of a 'normal' mage's mana. The SG girls have mana much stronger than a normal mage, but even their density pales compared to 'Shadow-sama's".
His mana is literally so dense that the magic canceling relic can't actually do more than weaken it by a small amount. He still has most of his power, lol.
Any energy that is shown from anyone else is active magic. For instance, a fire spell. The mana can't be seen, but the flame can.