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 ...
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.
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 ...
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 ...
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 ...