Once a character gets slip damage, it will be damaged continuously for certain amount of turns. An icon appears by the head of the character getting slip damages.
Some characters, such as Itachi Uchiha ( A Cold-Hearted Criminal ), can cause slip damage by using Ninjutsu.
Some characters, such as Kabuto Yakushi ( The Sound's Spy ), have skills to decrease the chance of getting slip damage.
To heal slip damage, switch the active character which gets slip damage to a character in back row. And then the character will not have slip damage from next turn even it is placed in a front row again.
Itachi Uchiha (Beyond the Sharingan)