Certified GKH Gardening Expert
I do not see any signs of bites, or being eaten but I do see the start of a fungal infection.
That needs to be in a cactus soil, which is designed to dry out very fast. What it is in now will ALWAYS lead to an infection, 100% of the time. In fact, I have put many cactus in potting soil on purpose so that I could snap photos of the resulting infections to show how detrimental a heavy soil is on a cactus.
Just be sure to get that into a lighter mix, and let it dry out completely between waterings. Ideally, it will get to the point where you only have to water once a month or so, and it will be thoroughly dry between waterings.
You may also want to replace 1/4 of the water next time that you water with peroxide. This can help kill off the infection, but only after it is put into the correct soil.