there is 2 problems i have with 2 diffrent cactus
1 i have a cactus that i had about a year now and on one side is a copper color iregular shaped mark almost the size of a penny. its not rot because its not soft. its in a pot about 4 or 5 inches deep and 3.5 inches wide. i water about once a month
2 this cactus i had about 7 years now and has been fine up to now. during the summer one side of it is turning brown. its loosing its green and looks like its shriveling up. could this be a sign of lack of water.