He was the bitter enemy of the Second Tsuchikage, the two killed each other during battle. He was resurrected along with other deceased Kages by Kabuto to fight in the fourth ninja world war. One of those Kages is the second Tsuchikage; and Mizukage would had attacked his bitter enemy if not for Kabuto's control, that doesn't stop them from arguing though.
When Gaara and Onoki arrived to fight them with the allied ninja forces, the Mizukage urged them to defeat them quickly so that they can rest and offered to reveal all of his abilities and weakness for them to do it.
Gaara caught him in his sand and placed seals on him over the sand, but Mizukage revealed a water-like ability to release the seal and he busted free. He and the 3rd Raikage then plowed through the alliance and he summoned a giant clam. Ninjas tried to attack him but their attacks went right through, he told them that the clam was creating a mirage and it's useless to attack him and that they have to attack the clam.
After the Mizukage was making short work of the alliance army, Gaara and Onoki came to fight him. Mizukage's abilities were proving difficult for Gaara to handle, but Gaara used his sand to find where the clam was hiding and Onoki destroyed it. Mizukage apparently killed Onoki, but this was shown to be a trick by Gaara. The real Mizukage was then trapped in a giant sand pyramid by Gaara so that he couldn't use his water to escape, but he did escape using his most fearsome jutsu.
Mizukage causes the oil in himself to heat up and he exploded, he then reformed himself and created a child-like duplicate of himself, while he was left in a weakened state his duplicate is able to repeat the process over and over again. It's incredibly quick and forms a water blade on it's arm. Gaara was able to find Mizukage and capture him again in his sand, but the sand was proven ineffective and instead the Mizukage decided to use it for protection. Gaara and Onoki asked for help to try to defeat Jokey Boy, but Mizukage refused, saying that he was instead going to help them by letting them help themselves; if they can't defeat him, how can they defeat their real enemy? Gaara got the idea to carry his father's gold in his sand and wrap it around the super-heated Jokey Boy, causing the gold to melt around it and harden.
The 2nd Mizukage has stated that he's a genjutsu user and is a member of the Hozuki clan, same as Suigetsu. He has the Hozuki clan secret jutsus, such as Hydrification which turns his body into water, and Water Pistol where he shoots a high-speed ball of dense water from his finger capable of tearing through bodies. Unlike Suigetsu, Mizukage's body is mixed with a oil. His ultimate attack is the Steam Devil Tyranny, with this the oil in his body heats up and causes him to explode, he then reforms himself but creates a duplicate of himself. This duplicate is capable of heating itself up to explode and then reform itself over and over again. Using this jutsu however leaves Mizukage weakened.
He's also able to summon giant clams which are able to produce a mist that creates a mirage-like avatar of himself that can attack, but cannot be harmed. The clam also uses this mirage to hide itself.