venting system built into it as a safety measure. All sealed batteries will vent hydrogen as a byproduct of the chemical reaction inside the battery. It's not necessarily an indication that something is wrong. I do think that your corrosion coming back within a week is very unusual though.

Excess venting from a sealed battery is sometimes an indication of an alternator problem. The general rule of thumb is you get excessive corrosion on the positive terminal from excessive overcharging. You get corrosion on the negative terminal from undercharging. You get excess hydrogen venting from overcharging.

Take all of that, and if you have excess corrosion rapidly reforming, it would normally be a sign of overcharging, and you'd expect it on the positive terminal. Because it's happening on the negative terminal, you don't have a clear troubleshooting sign to work from. I'd clean it up and use terminal sealer on the terminal after you put your clamps back on and let it run for another week to see what happens.