Movie with an alien that spits acid

In 1999 i watched a horror movie on TV, i was a kid and i was scared so I didn’t watch the whole movie, but the scene I saw was etched in my memory.

It’s possible that what I described down happens in the first few minutes of the movie.
It was a rocky desert and a team was looking for something in that desert, there were military personnel and scientists. Then an alien (it was a gray type or similar) appears on the screen, the alien was hiding in the rocks and spying on that group of military and scientists, the camera zooms in on that alien’s face (a typical zoom in on a character’s face that was characteristic of films from the 70’s and 80’s).
So in the next scene one of the scientists separated from the group appears researching something, and then the alien appears to him and spits an acid on him that starts to melt him, he screams and the team goes to the place.

At that moment i got so scared and stopped watching. Everything I described happens during the day in that rocky desert.

I research this movie for years, and wasn’t any of the classic alien movies.

Maybe Xtro 3? Just guessing