You are quite right it will be mould. The bes may have cleaned them up as the temperature increases, but they may not use the pollen if particularly damp and moulded.
Insufficient ventilation is the almost for sure the cause here. You have kept the OMF closed completely and used the 'famous'(or infamous) matchstick method to provide ventilation. A very poor choice.
Open, or part open, mesh floor and no top ventilation is sooo much better. Allied to top insulation to reduce thermal losses and condensation is the way to go. You may well find, if you had the porter bee escapes uncovered last year, that the roof ventilation has been completely (or nearly so) propolised.
This should indicate to you, as a beekeeper, that the bees abhor top ventilation!
With solid floors ventilation is paramount and a fine balance between damp or dry and warm or cold.
None of my hives have any top ventilation over the winter - there is not any in a tree nest, after all. I insulate the top - a tree nest usually has considerably more than 6mm cover over the nest. The moisture in a tree nest will be absorbed by the tree more so than in a man-made wooden box, so ventilation is required in the man-made box to avoid excess moisture. There is often a large free volume under the nest in a tree compared to a man made box. Bees naturally live in trees. It is much better to treat the bees similar to a natural nest than propping up their roof to leave it draughty for the whole winter.
They may be short of protein for spring build-up, but apart from that removal may not be too serious. As long as they are able to source sufficient protein they will be OK and build up normally.
Think about if you had left the frames cold way - there could have been pollen on more frames at the front of the hive. Those of mine which I have given a quick check recently are all dry. I expect the same in all of them.
RAB
