Ive found a neat way to attach moss. You will need to have a hairnet, and a small-medium piece of moss.
first You will place the moss on the pot, secure it down by putting the pot (with the moss on it) in the hairnet.
Shortly, in time, the moss will start growing on the pot. You can take the hairnet off the pot once the moss has a good growth on the pot. If you think that taking the net off will mess up the moss, you can keep it on, in time, the moss will grow over it.
Some mosses that you can do this with is christmas moss and java moss.
As far as attching any other plants to it, Im not sure. Ive had of tying plants on driftwood with fishing line, but Im not sure how that would work with a clay pot.
Make sure before you add the clay pot that It is not glazed. Ive heard of glazed pots being toxic in aquariums