I cannot say for sure, Athiril, but you could ponder this:
From the front, that third element just might unscrew. This would necessitate drilling two holes, 180 degrees apart, on the metal ring in order to be able to put a spanner wrench (or pair of pointed scissors) to the task. Or, you might attack the problem from the rear. You might NOT have to actually remove the fungus element; you could judiciously, with long tweezers, rub the affected area with either glass cleaner or lighter fluid on a clean paper tissue after, of course, removing the rear element.
Again, these are just thoughts that I would ponder and then maybe attempt. I do know that this particuar PK mount Takumar is more difficult than most Taks to disassemble. - David Lyga