your problem is more general than just turning off movement while crouching. you need a solution that allows you to change the controls based on the state of the character. when a character is crouching, they have some things they are allowed to do, which are different than the things they are allowed to do while falling, or swimming, or dying, or being knocked down, or during a cutscene… you need a state machine.
a state machine is just an enum variable that controls the flow of execution with a SwitchOnEnum node. it allows you to make different rules for each state. you can use multiple enums to represent more combinations of states, like using 1 enum for physics states, and another enum for attack states. with a state machine, you can do things like not allowing the character to crouch while falling, or not allowing the character to shoot fireballs while swimming.
in your case, in the crouching move state, letting the input axis MoveUp reach 0 or more, results in setting the move state to standing. in the standing move state, letting MoveUp go below 0 sets the move state to crouching.
