It's powdery mildew.
lower your humidity.
It also looks like you have or had spider mites
spray this stuff to kill both, but follow directions carefully. test on a small bud first.
https://www.icmag.com/ic/showthread.php?t=18473
also get more direct light on your buds. it prevents p.m.
If I see your picture correctly....there are webs on the bud....you have been invaded by the Borg...aka spider mites (check under leaves for teeny black/grey/white specs that move to confirm). If it is spider mites.....then at this stage....Floramite/avid/forbid....try other stuff if you want (drop temps/cold water spray/neem oil/wash down/co2 drown/sulphur/predatory mites/ladybugs/hot strips/bombs/other homemade remedies as suggested by the previous poster) but now that you have webs you have to move quick or they will take over all buds...Floramite would be my first weapon. These are strong chemicals....use as directed...I don't use them in flower but know people that use them in first 1/2 of flower. With mites prevention treatment is critical! Half measures at this point will lose the entire crop. Make sure to repeat treatment every two days and treat all plants in the room...the Borg will not go easily. Spray plants at lights on and make sure to get under the leaves....but leave Lights off until they are dry (will be like a cloudy shorter day to the plants).
No new clones hit my room without two weeks of dips/spray in quarantine.
And if the white on the leaves is not frost from crystals then you also have powdery mildew....use Eagle20 to solve that....mites are the priority.
Good Luck!
Thanks, Bigge. These were grown from seed but they are in an outdoor shed. I have had the Hot Shot strips up and hadn't seen any signs of them returning until now.
I'm going to mix up the solution described above and get it on them asap...
GP
sorry to hear of your woes GP...but it does look very much like mould. not salvaging what you can by harvesting now can become a game of diminishing returns later on...
i haven't flowered indoors for years, but when i did i was able to control mould by using pure neem oil mixed with silica and seaweed concentrate. basically 3mls neem oil, .2ml silica and 3mls seaweed concentrate/litre of water. i sprayed every week during lights out up to week 6 of flower in a very humid environment as a preventative. the benefit of using neem oil is that it keeps the bugs off your plants as well...
http://www.organicneem.com/neem_water_soluable_uses.php
wish you all the best...good luck