Cactus turning brown. Kept under a grow light all summer. Now in a bright window. Don't water it much.. only when extremely dry like every couple weeks.

What could be the problem?

That pot is HUGE and the Haworthia (or hybrid) looks very happy. Makes me think too low light and too much water as Haworthia like less than full sun and more water than a cactus. Grow lights, unless you have to wear your sun glasses when they are on, are no substitute for sunlight.

Your potting soil looks like compost, way to organic for a happy cactus. The pot should be just larger than the root ball. I suspect a 6 inch pot would have been a better size. Hopefully, its not too late to save the cactus.

We received advice that the cactus was rotting Due to over watering and to cut the stems above rot and stick them back into cactus soil (which is what we used here. We buy cactus soil at Home Depot) and hopefully they will grow roots..

So you cut them off? Where did you get that advice? What did they look like inside?

We received advice that the cactus was rotting Due to over watering and to cut the stems above rot and stick them back into cactus soil (which is what we used here. We buy cactus soil at Home Depot) and hopefully they will grow roots..

They were mushy and brown. We got the advice from a friend who has lots of cactai. So yes we cut them above the "rotting" spots and stuck them in soil. No idea if that will work but we thought we needed to take some sort of action quickly