Any extra columns you set will remain until the app is uninstalled. If you wanted to clear the value, you could use the Set an Extra Column Value on the Current Record Action again, but set a blank value.
If you want to set a value that disappears when the app restarts, you can use the Set a Variable action and select a data source. That behaves similarly to Set an Extra Column, but the values are cleared whenever the app is killed, and instead of using ds: expressions (as you would for values set with the Set an Extra Column action), you use dsvar: expressions.
So, if you were to use the Set an Extra Column action to set the column "foo" on cloud collection called My Cloud Collection (Cloud Collection), you would access that value later using the expression "[[ds:/My Cloud Collection (Cloud Collection)/record/foo]]"; whereas if you were to use Set a Variable with variable name "foo" on the same cloud collection, you would access that value later using the expression "[[dsvar:/My Cloud Collection (Cloud Collection)/record/foo]]".
The expression editor doesn't have a shortcut for entering dsvar expressions, so the easiest way to enter them is to select some other column from the data source, then modify the expression swapping "dsvar" in for "ds" and changing the part after the final slash to the name of your variable.