Interesting that it made so much difference. I use a quick spray of kitchen surface cleaner in the water usually and it does help, but I don't get as long as six months unless I was careful to store the palette in the fridge when it wasn't in use.
Just to check, when you say soap do you mean dishwashing liquid or something else?
Wouldn't you really have to boil it for a few minutes to make a significant difference? I'd imagine the palette paper is the least of the problems anyway, especially if you have a bleached one. It's not going to be sterile of course, but not teeming with bacterial either - boiled pulp, bleach, hot rollers for pressing. Plus it has to be food-safe, so presumably there are some standard they have to adhere to in terms of how clean the stuff is.
Yep, I think that's a bacterial matt forming. Pretty sure it's not a slime mould, you'd know if it were!
Einion