Okie dokie
The answer is yes they should be turned over once simply as a precautionary measure when exposed to air, this will allow any trapped air that the clam can't clear manually to escape. The clam will do the rest, that being said always ensure a clam is as closed as possible before moving to ensure that they don't get over exposed to the air, I move my large t. maxima and didn't turn upside down due to space restrictions but I made well sure she was completely shut before lifting her out of the water
You params look and as long as they are stable you should be fine
Make sure you keep an eye on temp as they will close up if it's not right!
The next thing is can we see some pics and species of clam is it? How long did you acclimatise it for?? Given that it's opening (even if not fully) that's a good thing, especially as it's reacting to light changes. The main thing is to let them settle before disturbing them again as you don't want to over stress them!
How frequently is it closing up? This is a normal reaction for a clam clearing debris or detritus from themselves and if they're continually doing it it might be a sign of an irritant inside the clam.
Some pics and info on the clam will help the most in solving if somethings wrong or if it's just taking it's time to open fully