Bleeding rads. the little white section is not moving on its own - whole fitting is

Hi
On some of my rads when you put in the key into the section of the rad to bleed the rad and twist, the whole fitting in that area of the rad is moving and not just the little white section with the the air release hole on it.

What can i do to prevent this or is it any harm. Know i could probably put a wrench or pliers round the bigger screw fitting to stop it moving but dont want to do that off hand in case it might damage the fitting

When the while section starts to move water starts coming out of it

Dont want mess round with it too much in case i damage something

advise?

Put a spanner on the larger section to stop it moving. Then open the bleed screw. You won't do any harm. Just use a correctly sized spanner or an adjustable one so you don't round the edges.