Flatten a q-tip to get it inside to wipe the stuff away. You might just find it has been etched by the previous juice(s), and cannot be removed. If that's the case, you should be ok.
Another cleaning tip (I have never used) is false teeth denture cleaning tablets. I know they do a great job on my teeth.
Just use about 1 cup of warm water, drop both the atty and the tablet in, let it bubble down, and again, shake, shake, shake.
Leave it overnight - I do.