I’m new here but hoping for some help. Is he going to die, am I watering too much?
I planted seeds around 1.5 years ago and I’m amazed how big he’s grown but recently he’s been turning yellowish brown
Can I save him? Thanks Amy
I don't see any yellowish-brown. Consider taking an image where the cactus is not backlit, highlighting the areas of concern.
That said, there are obvious issues here; when there is moss growing on your cactus soil you're using the wrong soil and it's too wet. Try to find some cactus and succulent soil and repot.
Does that pot have drainage holes? If not, repot it into something that does.
The moss and general shape indicates that the plant doesn't get enough sun. This species grows in full sun in Phoenix, AZ, so it will take a lot of sun.
I believe this is an Echinocactus grusonii.
All of that said, it doesn't look too bad. Get your soil/moss issues resolved, give it as much sun as possible, feed it occasionally and it should turn into a nice plant.