It was supposed to be a stalagtite that Kirk broke off, but it looks like the prop department made it appear extra "penisy"
I bet they had a good laugh about it in 1967.
That they "got away" with showing it on TV.
Yes, I’m sure it was deliberate, intended as humor.
Captain Kirk Wields a Giant __nis - Star Trek: 2min
https://www.youtube.com/watch?v=ql8KzgSdgVM