It's the condensation draining out of the evaporator out of your heater box, not sure on the Seat but most cars have a rubber drain pipe that comes out of the floor/bulkhead.

The smell will be from mould/bacteria in the evaporator, try running the air con flat out with it on hot for a while, may help, or get some air-con treatment which is a spray and follow the instructions on it's use. Wynns do some stuff in an aerosol I've used before. You could have a breather pipe split somewhere and getting fumes from that also, I know the 1.8Ts suffer from the pipes around and under the inlet manifold splitting.

Regarding forums, if it's Seat Cupra net, it's been going 10yrs or more, bet the same questions have been asked time and time again, worth searching first. Most people will probably look at a new post and think, sod that, it's been asked hundreds of times and not bother replying.