I ran into an interference between the plugins named in the topic. I have a working environment based on Neon.0, Scala IDE 4.5rc2 and EASE 0.3. But when I updated to Neon.1 the Scala sources are not recognised anymore (no syntax highlighting). But the real reason seems to be my installation of EASE (0.3, but I also tried 0.4 nightly). When I uninstall EASE after the update to Neon.1, Scala sources are back to normal. I posted an information in the EASE forum, but may be it is related to Scala IDE internals, so I thought I leave a comment here, too.
By the way: the same effect with EASE is also true for Oxygene M4 - Scala works fine without EASE but has the same problem, when EASE is installed (in Oxygene only EASE 0.4 can be installed).