Euphorbia struggling, what to do?

Hello all, I bought this Variegated Euphorbia Trigona this past December and while it hasn't died, it has not grown at all and when I was adjusting it as it was tilting over, i noticed the brown part of the stem. Is this a dried-up root rot? or is this just normal callousing?
Also, any recommendations for watering / potting one of these guys?

I am no beginner with houseplants may I add, but when it comes to cacti / succulents my confidence in my ability plummets lol.

Thanks so much!