One cause nobody has mentioned and it is worth checking your notes for: kahm yeast. That flat white film that forms on top. It is not dangerous in itself, but left to grow for weeks it puts musty bitter flavours into the top layer of the mash.
So keep everything under the brine and skim it early rather than deciding it is harmless and ignoring it. Separate point: bitterness alone is a flavour fault. Fuzzy mould, or pink or black or colourful growth, or a rotten smell rather than a sour one, is the other conversation.