the slime you mentioned is similar to the slime coat in fish. Apple snails will produce "mucous like" slime when there is something in the water that irritates then. It could be trace of something that was on your hands, or ammonia, nitrite/trate etc.
It is normal for them to be more active after night. The food... is hit or miss, of course mine were used to snacking on my pennywort.
They do prefer ph of 7.6 or higher. In lieu of that calcium supplements are helpful. Plain old calcium tablets (like you get in the vitamin section of a pharmacy) work well. Some people have used tums, but I just didn't think it was a good idea. High calcium foods work well. Blanched dark green veggies like romaine, vegetable greens... spinach. All high in calcium.
Applesnail.net has so much information on apples. I highly recommend it. They even have do it yourself recipes for making snail food or "snello" or snail "snax".
My snails never got to enjoy it, the girls snarfed it, and what they didn't get the shrimp went crazy for.