Welcome Johnny v to our little nutty corner corner of www. I am not an expert on your
cactus but spiders do live around them as well as other
plants. Find out what type spider and call a local botonist if you can. I have one as well as a
cactus cutting from my mothers
cactus. I have them in big (2qt) clay pots I throw a little water at them every week or two about half cup each. Had them inside all winter they are now on my porch have not seen anything going bad with them. I would almost bet yours has been over watered or somehow roots may have gotten root rot check that out.
Again welcome to the forum.
Jimmy
They are so tiny that you can't see them, but they spin webs and the leaves can become bleached out, discolored and start falling off.....