Drive-side Ultegra crank arm weirdness – is this dangerous or cosmetic?

I was doing a hard uphill (virtual) ride last night with a lot of out-of-saddle riding. I noticed my right pedal felt increasingly odd and thought that the pedal might somehow be coming off. After the ride I had a look and found what’s shown in the two pictures at http://jon.es/other/ultegra-crank-1.jpg and http://jon.es/other/ultegra-crank-2.jpg. What looked initially like just a cover was quite a bit further off than is shown in the images. At first I thought it was just a cosmetic thing and so I tapped (firmly) with a hammer on the 4 stubby arm-like sections that were coming away from the chain rings (the top-left arm in the 1st photo shows a pretty wide gap). They went back in to some degree, but as you can see in the photos that they’re still not flush with the outer body of the rest of the front rings. But then I thought “hang on, if that’s just a cosmetic cover, why did your pedalling start to feel so wonky & messed up?”  So now I’m guessing that that central 4-armed piece is actually integral to the whole ringset body and that maybe I’m in danger of having it completely break apart.

Has anyone experienced this before? It’s Ultegra 6800, I believe. I’ve done over 20,000 km on it, so it’s not exactly new. I changed the chain rings a few years back, but as far as I remember that didn’t involve changing the part that now looks like it’s about to break off / fall apart.

Thanks for any help / suggestions!