I think they are indeed sponges but like said above monitor, they shouldn't move lol. I have several in my sump that have this appearance. Best of luck :)
Ciliate or cotton sponge. Simply scrape off in a container of tank water before they cover the stem as calcium is a necessity for euphyllia and you dont want stem smothered
My torches have the same white patches on the stems too and a lot.
I took them out and inspected closely. They are not growths, seems only white pigments and no way to scrap off.
Overall the tentacles open wide and heathy.