Thanks for the replies.
It’s taken me some time to get back in the super, but I did remove the frame yesterday and have attempted to clean the mood spots as suggested both on the hive and off it.
Those that can be reached wipe straight off.
The mold spots are ONLY at the end of the channel in the sections around the tube hole, which are recessed back and so almost impossible to get at. The bent over end of a pipe cleaner does work to some degree, but the gap at the top of the tube is very fine and so not easily penetrated.
I doubt these marks are propolis as I am not sure how the bees would get propolis in there but would be happy to be wrong, if anyone cares to disagree.
I have soaked the whole channel in a very mild bleach solution for 15 mins, then washed it thoroughly out, Then soaked the end in 50/50 apple cider vinegar and warm water.
Photo is post cleaning efforts. Hmm…