You must have a link to your "http" shoutcast page on your web. Or someone else does.
This probably isn't anything evil. The spider in question is asking for your robots.txt file, so it's probably not an evil spider.
If it was a "bad bot" it would ignore the robots.txt file anyway. And probably not even look at it.
Sounds like a search engine found your html page shoutcast page. If that is bad, change your port and if you want the page to be hidden from search engines, put the link to your shoutcast html page in a separate directory. Like /norobots/myservers.html
In that directory (/norobots) also include a robots.txt file contains two simple words.
"no cache" will also prevent Google and others from archiving your pages.
If you wish to conceal links that have already been spidered, change the port of your shoutcast server.
If you go to alexa.com, you'll find some pretty good documentation on robots.txt.
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