Why is the stalactite covered with blood before Gabe lifts up his opponent against it to kill him?

It isn't covered in blood. It's just the lighting, or the prop has been stained.

To make things worse, we can view the red stalactite for a second or so, which is quite long. At least, it is probably too long to be caused by a mistake; therefore, I believe that there must be some sense in it.

If it was a mistake and noticed in editing, it is far too minor to justify the cost of a reshoot. Cliffhanger is not exactly concerned with accuracy.

However, I agree there's some sense to it. I think it was done deliberately to make the stalactite stand out against the grey background.

At 2:35 we get a shot of the pink stalactite.

The point of this shot is to grab the audience's attention towards the stalactite. Let's consider what this shot would have looked like with a grey stalactite. My hack job is below.

The shot doesn't work, what are we supposed to be looking at? I believe they deliberately tinted it. Alternatively, they did several takes of the impalement, the fake blood stained the prop, and they found the pink-stained version worked better.

After a cut to Kynette's face, we're in a different shot showing the impalement about to happen. Now it's grey, a continuity error, but only for a moment. This shot was likely done at a different time and location, possibly with a different prop, or possibly before they decided to tint it.

Then, goosh, there's the blood.

There is a far worse continuity error in this scene. Just before impalement Gabe lifts Kynette, no stalactite.

After the lift, which is all done with the arms, it's right in front of Gabe's face.

And then it isn't.

Bear in mind, that I can only work with the screenshot, since the video clip is blocked in my country.

There are 3 possible reasons that come to mind.

1. It is a mistake. They inserted a clip with the fake blood on the stalactite before the kill scene because they forget to shoot it without blood. We can safely rule this one out, as this mistake could have easily be corrected in the colour grading stage that comes after the cutting. The scene has mainly blue light, so they could have reduced saturation of the red in post production easily. It is highly unlikely that everybody involved overlooked it.

2. The stalactite is covered with iron oxide, which can also be found in human blood as part of hemoglobin. It is causing the red colour. If a small iron ore vein or deposit is directly above the stalactite and washed out by water, then it's possible to have only one stalactite covered with it, not the rest. Update: after reading Schwern's answer it is clear there was no natural iron oxide on the stalactite as it is also shown in grey colour.

3. It is intentional. Painting it red makes it stand out as something evil especially when you have blood in mind. It is a foreboding sign that something bad may happen. There doesn't have to be a logical reason for the stalactite to be covered with blood before the scene, as this movie is fictional. It is an artistic means of the creative arsenal of filmmakers when they decide to include allegorical signs even if contradicting logic. I never saw an action movie in my life that had not at least a few scenes that were effective but in conflict with the laws of physics or logic. But with reason no. 2 in mind, a naturally red stalactite is theoretically possible, so it's somewhat convenient as it offers a logical explanation, leaving the viewer in an ambiguous state of wondering whether it's natural or an evil sign.

Stalactites are formed by the deposit of minerals over time. Depending on the makeup of the minerals, they can vary in color.

To quote this article by the National Park Service:

Most scientists believe that the color of speleothems are determined by the mineral content. Pure calcite is white and almost colorless. Iron and other minerals, as well as acids from surface vegetation, combine with calcite crystals to add shades of red, orange and black to the color of speleothems.

In the meantime, I did further research, and it could very well be a mistake. In IMDB, they even call it a goof:

When Gabe lifts Kynette to the stalactite, we see that it's already covered in blood, even dripping, before he pushes him up, piercing him to death.

The link is here.

Now I am very disappointed. I didn't notice any of the other mistakes in this movie, because I usually want to enjoy movies and relax watching them, and thus don't look for such mistakes. But that scene has always worried me.

I guess that we will never find out what was really happening. I believe that the other answers are better than this one, but I wanted to bring another opinion to peoples' attention.