LX Keyboard arm won't stay the latest position

[hulk]

keyboard tray of LX Keyboard arm won't stay on the position I set it to -- it always returning back to the highest position.
I know I need to tighten the 4mm screw, I did that, though.
it's hard to move the tray down, but if I realease it, it will go back up. As well some white liquid (a grease?) has appeared near to the screw.
So now I'm afraid of overtightening the screw and breaking the arm.
Is that a manufacturer issue? Or, maybe I need to tighten it more? How to know the maximum of tightening I'm allowed to do?

[hulk]

oh, sorry. I needed to loosen that screw instead of tightening... Now it's OK.
so what is that white stuff?