Fungus of the stick. Logitech F310.(Gamepads)

Greetings inhabitants of the forum. I have a gamepad, he really already 3 years old but works as new, serviceable.
But here's the thing, there is a problem with a fungus on one of the sticks, that thing which is put on the protruding from the mechanism of the stick canoe(I apologize in Advance for my ignorance of terminology), black fungus, which is actually in contact with my thumbs, in a nutshell, it is no more. And I wouldn't worry, if it bokowski gamepad or PS3/PS4, fungi to these gamepad you can easily order online but I have a Logitech F310, and I was not looking, there are no fungi to them. The same canoe sticking out of the mechanism of the stick, that and hboks that the PS3/PS4 are totally different.
Here is a photo of the mechanism stick with protruding from it canoe.

Here's a photo of the missing item.


Does anyone know whether it is possible somewhere to find a fungus under it.
And even if not, are there any ideas how to ignore this problem? The controller, as I wrote above, 100% working, there is just not enough one plastic thing.
Thanks in advance for your time and possible answers.