Locally someone here was going to test these but Iím always wary on strip tests.
My guess would be subtle water quality issue eg tank meds = slightly unstable tank, low oxygen & CO2
maybe generally ok but borderline if something else happens. Or slight ammonia.
Water change fixes.
If it happens again after feeding the worms, maybe a quality issue (or ammonia issue as mentioned above). Ammonia test kit will tell.
Doesnít sound bacterial, fungal or parasites so brings me back to something going on in tank water column or something recently introduced as a thought.