Help! my cactus is turning yellow

[livscactus]

Hi everyone, it is my first time posting and I'm so sad I have to! I have had this cactus for years... and he has been in my room by a window. Well recently I looked at him and the bottom of him is turning yellow! I poked it and its not mushy, just a little softer than the top of him. I don't want to lose him so can anyone suggest how to fix it?! I don't even know what kind he is... I'm such a bad cactus mom. I have also included my other 2 pots to see if anyone sees anything wrong with them. The one is turning purple at the base and the other is very tall. Thanks in advance!

[EmPen]

I am also having an issue with yellowing cactus. I'm also not sure why the cactus grows so thin. I've had this cactus for a few years, when I got it it was short and stocky, and it grew fast but thin. Now the growth has slowed down and it is yellowing at the base. Any thoughts?