The "fluff" is the result of acid reaction to the lead (or whatever is used these days) terminals. Normally the battery vents through a tube at one end, but only a little. Yours suggests that excess gases are escaping, which indicates a dying battery, or an overcharging alternator. Depending on where you are in France, as the weather gets colder, the battery will suddenly give up if it is the problem.
Main concern might be where else is being attacked by the acid.
To clean it up.
Remove battery.
Wash the terminals with warm water. A weak solution of bicarbonate of soda might help. Wear good rubber gloves.
Wipe the cable terminals the same way.
Might be an idea to wipe over the whole area under the bonnet with the bicarb solution. Careful not to drown any wiring connectors, especially the ECU.
Once refitted, with engine running, get the charging voltage checked. Should be around 14v. Anything over 14.5 is potentially damaging the battery, and may cause bulbs to fail, and damage electronic components.