Take a look at the fifth paragraph of this article, it says the following:
To test the effect, physicists needed to compare two clocks - one that is stationary and one that moves.
Why is it moves instead of is moving? What justifies the use of the present simple here?
In your quote:
one that moves
is only stating that this clock has the ability to move. Since no action is specifically referred to in that paragraph, is moving is not used.
one that is stationary
is OK because that is the state of that clock.
'To move' is a verb and 'stationary' is an adjective. 'Is stationary' is simple conjugation. 'Is moving' would be present continuous conjugation.
Simple is preferred unless there is a reason to use present continuous. The normal reason for the latter is to emphasize timing, that something is actually ongoing.
So for the moving clock simple suffices and there is no reason to turn to present continuous.
