When you find yourself trying to remember a show (or any works) that's on the tip of your tongue but just out of reach, come here - the collective brain of the TVTropes community can probably help. Post all the details you can remember (examples help). If you're looking for a trope, head over to Trope Finder. Have general questions about tropes? Visit Ask The Tropers!
The Forever War sequel has some shapeshifters like that.
Edited by Omeganian
Maybe 'The Stonewalkers' by Vivien Alcock?
Doctor Who has the Weeping Angels, aliens that are "quantum locked", i.e. the second they're observed they become a statue but can move very quickly otherwise, literally in the time it takes to blink. They first appear in Blink. I just wondered if you'd come across them in one of the expanded universe books, e.g. The Weeping Angels of Mons?
All I remember about this series are these aliens who can stand still for a really long time and are often mistaken for statues.
I don’t know either the author or what year the series was originally published.
Please help me find out what it was called!!