I’m not so proud of it but I was pretty sure that they would think right side orange square is the scroll thumb!
Hmm, perhaps add arrows?
It’s really huge for a scroll thumb. But also, on touch devices it’s expected that you scroll by dragging the content rather than by dragging the thumb slider. The slider’s main purpose should be to indicate your position within the scrollable area.
Also, you should really try and keep things like what’s in your screen shot together on a single screen. You shouldn’t need a scroll view just to get a username and password, you should be able to fit both (and the “log in” button or equivalent) onto the same screen relatively easily.
The style is great, but the functional design needs work.