I figured it might be a conch by the shape of the shell, i've been looking through hitchiker ID's and that was the closest thing i could come up with. I haven't seen a foot on it either but i've never seen it move. I saw it a long time ago when it was a lot smaller and on a rock but just thought it was an empty shell, this is the 2nd time i've seen it. Could it be just some sort of a snail?
And as for the yellow stuff, i think its just some sort of a sponge, not really sure myself. I had a Sixline in my tank for quite a while and he never really bothered it.
Thanks,
Jeff