For a while I was confused by these broken bits of coral, that I frequently found at Arkona, and called them Alveolites goldfussi.
My friend Joe K. pointed out to me that Alveolites generally does not form branching structures but Cladopora does. I am now labeling these pieces as Cladopora roemeri as that is the most common species found within the Hungry Hollow member of the Widder formation. Besides the branching structure of the coral the calices, where they individual polyps lived, are shaped like little triangles (or sometimes I call them "fish scales".
There are many species of this coral and some were once referred to as "Coenites". Here is a piece that shows evidence of branching.
Most of the Cladopora that is found within the Hungry Hollow member of the Widder formation, which is Middle Devonian (Givetian) in age, at Arkona are small, inch or so sized pieces that have eroded out of the rocks. To find larger specimens you need to search for them still embedded within the matrix.
Joe K. managed to extract a large set of branches from matrix and reassemble them into life position as can be seen here.