What's wrong with this plant ?

What's wrong with this Euphorbia and what can I do about it.

 

If that is a hen's egg nearby, try keeping the chickens out of the greenhouse. The damage could be peck marks followed by disease in the damaged areas.

 

Has it had a mealy bug attack?

 

Snails used to do similar looking damage to my Christmas cacti when they were outdoors in the summer. A rounded largish hole that scabbed up. The damage in your Euphorbia is oddly bang in the middle of the stem though so has required some careful slithering up if snail was the culprit.

I'd cut the whole stem off. The yellowness has spread nearly to the top so it might not root, but it's always worth a try- just slice the top healthiest looking part off, let dry and pot in sandy compost.

Dilute liquit seaweed would do good to the remaining plant. I'm a new convert, the stuff seems to be very good for cacti and succulents during the growing period.

(Remembering that you have troubles finding your threads, I'm tagging you to create alert @Michael Hewett . I usually have two tabs open and sail between them, Home in general has the threads I like to read, and New Posts also covers Off topic etc. )