A sea star can look almost stationary from above while its underside is busy. The familiar star-shaped outline is only one part of understanding how the animal moves.
Look beneath the outline in a reliable image
NOAA explains that sea stars move using small tube feet on their underside and are not fish. Its sea star overview offers a starting point for separating the common name from the animal’s anatomy.
Use a labelled photograph or aquarium view to locate the feet. Do not turn over or remove a wild animal to make your own diagram. A view through aquarium glass can reveal something a photograph from above leaves out.
Describe movement at the scale you can see
If you watch a video, identify whether it plays at normal speed. Record what is visible: small structures changing position, an arm shifting or the whole animal moving relative to a fixed point.
Keep the identification as precise as the source allows. Different sea stars should not be treated as one interchangeable species. The exercise changes the question from “why is it doing nothing?” to “which part of the movement am I able to observe from this viewpoint?”