The gel is called aircond jelly, it is made of bacteria, mold, dirt and dust. There is no easy way to get rid of it, you will have to deep clean the entire unit. Once a year should be sufficient, more if your place is particularly dusty ie lots of stuff in the room. Trophies? Toys? Books? Collectibles? Anything that increase the 'surface area' that could be covered in dust and can't be cleaned regularly contributes to the dust build up.