Buttons sticking and "O" looks "melted"

The last time I used my controller about a week ago it was perfectly fine. Today I turned it on and the buttons were sticking (square sticking the least) and the "O" button, which looked normal last time I used it, looks like some kind of gel leaked out. It still feels like a normal button on top. This controller is for my PC and I am certain it hasn't been touched since the last time it was used and is completely OEM. Anyone know why they heck it would look like there was gel in the buttons? I looked a round but couldn't find any information. And should I just replace them with something aftermarket? Besides the sudden stickiness the rest of the controller is working fine.

The last time I used my controller about a week ago it was perfectly fine. Today I turned it on and the buttons were sticking (square sticking the least) and the "O" button, which looked normal last time I used it, looks like some kind of gel leaked out. It still feels like a normal button on top. This controller is for my PC and I am certain it hasn't been touched since the last time it was used and is completely OEM. Anyone know why they heck it would look like there was gel in the buttons? I looked a round but couldn't find any information. And should I just replace them with something aftermarket? Besides the sudden stickiness the rest of the controller is working fine.