I had this problem once before as well. The mucus is actually turtle waste. From what i have read/heard 80% of turtle waste is not solid. When it builds up, it needs to be cleaned and have a water change. To prevent the slime in the future you need to break the surface of the water either with a air rock or by breaking the water surface with your filter. I have Fluval 2 filter and i point the jet at the surface of the water to break the plain. I haven't had that problem since. Hope this helps..