Shot in one take, obviously no CGI. A scene that still dazzles today, done with an interesting use of filters that blocked out the shade of the makeup (red, I believe). Impressive and cool.
If you're in a hurry the effect begins at about twenty seconds in and it's worth seeing.
https://www.youtube.com/watch?v=4m3Rea0H_p8 (https://www.youtube.com/watch?v=4m3Rea0H_p8)
Cool scene!
The same effect was used in the 1931 DR.JEKYLL AND MR. HYDE transformation scenes!
That a nice trick, and one of the better octopus tentacles I've seen in older films.