Thanks for the replies guys.
I've had these since December, they are about 9 months old. The tank has been running since late October ... filter was established on another tank before transferring to this one.
I do a gravel vac and water change of around 30% weekly , and check parameters midweek just for peace of mind.
They have only just started doing it, and they only do it in one spot in the tank. In fact looking at them now, they are chasing each other out of the way to rub in that spot.
Their colour seems normal, and they are eating voraciously as usual.
The clowns are fine .. bouncing around as usual.
Maybe they are just quirky fish.