It said something about equip efficiency, but I can’t figure out how to increase it
The weapons aren't reducing your stats, red just means you're not getting the full value of the weapon. It's still worth using the weapon over nothing since you'll get *some* stats and gain proficiency.
Just equip an weapon to unit, and play quests with the unit. That's the only way to increase equip efficiency. Some of equipment like excalibur requires high efficiency.