This is benign; it is a known problem with XSD handling in STS, which is fixed, but didn't make it into 2.3.3.M2
If you can't live with the red squiggly lines, manually add the following entry to the XML Catalog...
Entry element: URI
Key type: Namespace name
(Just Add from workspace - browse to the schema and update the key as above).
And Project | Clean... | Clean All Projects
(You should also set the experimental option - globally - to load namespace handlers and XSD's from the project classpath - this can be found on Window | Preferences | Spring | Beans Support | Namespaces).
Quote from email w/ Christian 7/16...
this should be solved now. Unfortunately the fix didn't make it into 2.3.3.M2.
The version that you are using will load the XSD from the XML catalog always with highest priority and if not found in the catalog delegate to the classpath. This is clearly a bug which is now fixed. Please note this only affected the XSD resolution for schema validation; not loading of namespace handlers.