I'm unsure why the placeholder isn't showing in the module. I looked at the CSS and HTML and things look correct, but obviously something is preventing the placeholder from showing. Unfortunately, I don't know what it is. Looking at your page further, I see the search box at the top has a placeholder value set to "Αναζήτηση". That also is not showing, so it's not an issue with just our SCLogin module but something more with the site or template itself.
I'd recommend trying a different template to see if the placeholders work. If so, talk with your template vendor to see if they can help. If even the standard Joomla templates don't show the placeholder, let us know and we can investigate further on one of the standard Joomla templates which we know better.
Thanks,
Alex