This question comes up regularly from new and old members. Michael, Matt and Siham have summed up that which is usually said. I would just add that you (and every new member) should read the Guidelines and the FAQ where most questions are answered.
As you see below, the poems are not retained indefinitely so they are not "published". Alex, the owner of this workshop site, has provided tags for making the posts invisible to the search engines and if you use those tags they will not be retained on the Internet.
http://www.ablemuse.com/erato/view.p...#_Toc239789940
Publication Eligibility
We prune threads so that works posted on them do not count as “previously published”. This would render them unacceptable to some publishers. Please note that some publishers may reject poems that have ever appeared on the internet, even temporarily in a workshop. To limit this concern, Eratosphere restricts search engine access to workshops (see the FAQ section for more details). However, you should weigh this risk before you post.
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More about protection here:
http://www.ablemuse.com/erato/showth...ighlight=index