Use Debug Filter here

src/main/java/com/gmail/nossr50/skills/utilities/SkillType.java

@@ -81,7 +81,7 @@ public enum SkillType {
             }
         }
 
-        mcMMO.p.getLogger().warning("[DEBUG] Invalid mcMMO skill (" + skillName + ")");
+        mcMMO.p.getLogger().warning("[Debug] Invalid mcMMO skill (" + skillName + ")");
         return null;
     }