The 'Star Trek' actor had a strong reaction to the 'Curb' star putting his hands on the "Sesame Street" puppet on the 'Today' show -- taking to Facebook to write a lengthy essay that slammed LD ... while in the same breath explaining why it rocked him to his core.

He writes, "What the f*** is wrong with that guy? Elmo is, like, the best friend to multiple generations of children. In the Sesame Street universe, ELMO IS A CHILD, who is currently putting mental health and caring for others in the spotlight."

WW adds ... "And Larry F***ing David ... did ... that? And thought it was going to be ... funny? What? What an ass****. What a stupid, self-centered, tone deaf ass****."

Wil goes on to say that his dad used to abuse him as a kid -- shaking him by the shoulders and screaming in his face, among other things, and he says seeing Larry do that to Elmo had his heart racing as it dredged up internal trauma in his mind. He's quite serious, too.

He finishes by saying what Elmo means to so many kids -- especially in that segment, where he promoted mental health awareness -- makes Larry's attack all that much more sickening.

Wil ends with ... "A man who would belittle and mock that isn't much of a man at all. Shame on you, Larry David." This tirade of his, BTW, ended up being some 900 words on FB.

FWIW, Larry did "apologize" to Elmo shortly after his outburst ... and clearly thought it was all in good fun. Apparently not, though, from Wil's POV -- dude is straight-up outraged and hurt.