Maliwan weapons often have "Bonus Elemental Damage" listed in their attributes. I'm not entirely sure what to make of this; does it just refer to the listed elemental effect damage, which is presumably higher than it would otherwise be, or does elemental damage from these guns deal even further damage beyond the listed stat due to this effect? Is there an exact amount or percent of additional damage this attribute reflects?
It's an additional splash damage amount in addition to the weapon card damage. Percentages vary by weapon type.
It just refers to the elemental damage already described in the gun stats.
All written effects such as 75% more damage or 20% higher accuracy are already reflected in the written stats.
Added: Slag does not do damage on its own. It is actually very special because it doubles other damage dealt. Which makes one guy with a slag weapon genius in a 4 player co-op session. When playing alone slag weapons are generally not as useful, except against sturdier enemies along with switching guns.
Of note: in Ultimate Vault Hunter Mode (UVHM), Slag increases to triple other damage dealt. UVHM also increases slag-on-slag damage as well as weapon swap speed to encourage players (whether single or co-op) to use the slag-and-switch weapons technique for massive damage. This becomes extremely important with the enemies in UVHM having far more health and built-in health regeneration.