I saw your post very late while searching for something else. I had also experienced with that kind of pink slime and posted a thread maybe one year ago or earlier. Replies didn't satisfy. It had a phosphoric beautiful pink color but as you can guess it would not be too good to face with such a substance in your aquarium. I wasn't using any buffer then. It came out sticked with not growing Blyxa japonica roots. It formed up inside the substrate I mean. Since then it hasn't happen again. But I still wonder what it was. But I had suspected the iron tablets.