Haha I know why. It’s cuz I’m a dum-dumb 
The description of Manashield says “Negative Mana effects and effects that prevent special skill use ” so it makes total sense why it also resists Silence and Mindless Attack. Spell Slayer is considered an effect that blocks special skill use too, not a negative mana effect. I found out by donating a ton of food to science (rerolling) to find a Miki defense team: Miki – New August 2019 HOTM (Hero of the Month): First Thoughts & Discussion - #370 by DaveCozy.
And that’s why Poseidon casts two buffs 
One is to resist the new negative mana ailments and mana reductions, and the other is to resist ailments that block special skill use.