Fairy castle cactus - I have one that is about 10 years old, big, and it keeps getting black spots on it that I can't figure out how to get rid of. Any idea what is going on? It seems to have room for growth in the pot, and I not sure based on its size if I could find a bigger pot, much less get it in one.
If your cactus appears to have room for growth in its pot, it's best to keep it there for now. Here is more information on when to repot your cactus:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-repotting-info.htm
Check whether the black spots feel soft and mushy or sunken. This indicates there might be a rot problem, which can be caused by overwatering. Here is more information:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-going-soft.htm
Or, it could be a fungal disease, like those seen in the fungi section of this page:
https://cactiguide.com/cactipests/
Why is my Fairy Cactus splitting vertically?
This is normally caused by overwatering. You should make sure that the cactus dries out thoroughly between waterings. This article will refresh you on the care of these cacti: https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
The first thing that comes to mind is root rot from over-watering. Your cactus should be in potting mix designed for cacti. If this isn't available, mix one part perlite and one part sand to 2 parts regular potting mix. (Other recipes are out there; the point is to get a mix that drains freely and doesn't get soggy.) Let the plant completely dry out before watering to minimize the risk for root rot. I suggest you take it out of its pot and examine the roots for soft, discolored roots. Prune out those roots. Repot into a terra cotta pot which will help the plant dry out, further decreasing the risk of rot.
General information on fairy castle cactus: https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
this is apparently a very old cactus; it is about 7-8 ft tall in a 16 inch tall pot. it was standing very upright until it suddenly began "drooping" from the top and looking sick.
Check your growing conditions with the tips in this article:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
Is it showing other signs of distress in addition to drooping? This article details how to trim back a cactus, which is what I would recommend: https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-pruning-info.htm
The tip broke off
If it broke off in an accident, you can try to reattach it.
https://www.gardeningknowhow.com/garden-how-to/propagation/grafting/reattaching-broken-stems.htm
If it broke off due to disease, it's best to cut down to healthy tissue and let it heal over.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-going-soft.htm
I put my cactus plant outside one day to get some sun; when I came back home a lot of the stems were bitten off and I found some teeth holes on the main stem. I was wondering if the cactus will heal the holes up and be ok. Thank you.
The primary danger to your cactus is that with any wound, there's a good chance that infection or infestation will set in. You'll need to rinse the wound thoroughly with hydrogen peroxide, cut away any part of the tissue that looks rotted or darkened, and dry the area completely. You may want to dust the open areas with sulfur. Moisture of any kind is going to invite infection, so you'll need to protect the plant from water for a while. It's unfortunate that this happened in the winter because the plant will heal faster in the warm weather.
Here are two articles that may be helpful:
https://cals.arizona.edu/yavapai/anr/hort/byg/archive/cactusculture.html
https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
Can I separate my fairy castle mini cactus and plant in soil
yes. Separate the desired pieces and leave them on the table to dry for 2 or 3 days. Pot them in well draining cactus mix soil. Here is more:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/removing-cactus-pups.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/fairy-castle/fairy-castle-cactus.htm
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-repotting-info.htm