Yeah, go talk to Montblanc in the Elite Clan building. When he first gives out hunts, he has 2 prepared for you. The White Mousse is a Rank 5 Clan Mark so it will take some time and leveling before you defeat him if you aren't strong enough.

When you defeat him, you get a Sluice key to the Waterway where you get to fight the Esper Cucucilian or whatever his name is.