How to clean clear sippy cup seal??

So my 2 year old left one of her sippy cups in my husbands car & we didn’t realize that’s where it was for 3 days. 😬 She’s not usually in his car since it’s a pain in there but to switch out the car seat.

Unfortunately the last drink she had in the cup was her Lactaid (milk)... there was just enough milk in there to get the clear seal all gross & its almost like the milk had stained it with this milk film... I’ve scrubbed & scrubbed this thing, left it to soak in hot hot water... it won’t come off. I got the actual cup & lid clean no problem.

Is there a way to clean this properly or should I just chuck it? *she has other cups so she’s not using this one **

I’m not seeing an option to buy just the clear seal online but if you know where to find it let me know!