Clownfish diseased or ok??

Have 4 clowns and 1 Purple Tang in 100gal tank with sump, also inverts, soft
& a few hard corals. Water parameters are good, KH=12, salinity 36ppt,
nitrates around 5mg/l, Ca=440mg/l, Mg=1320 ... lots of flow, metal
hailides/actinics. Tank established for 11 months, clowns in tank for 4
months now with no prior problems.

Clowns are all tank bred Oscellaris. Over the last few weeks 3 of the clowns
have developed black spots all over their bodies (approx 3-6mm dia)
appearing somewhat mottled. They all sleep inside my sea mats/button polyps
(Zoanthus) so suspect this may be the cause. The other clown sleeps inside
my Clam (don't ask me why) but has no spots!!

Everything else is thriving .... any advice please??

John C