Hi Christian,
your query about your coral ID got my juices going. So whether you like it or not, here are some more bits of info (useful or otherwise)...
The coral skeleton above IMHO is that of a
Galaxea (another beginner's SPS). You also had another picture (the last one?) of a similar morphology. IME please do not throw those skeletons away. They may be alive. Keep them under strong lighting.
Galaxea skeletons have a creepy habit of regenerating their polyps after several weeks/months of appearing to be dead! Once it starts regrowing, there is no stopping it. This is relatively easy coral to maintain. So if you have the tank space, what do you have to lose?
IMHO this is a dead skeleton of a
Pectinia (probably
Pectinia Paeonia, the lettuce coral). Sayang, this would have been a beautiful coral.
It it rigid, as a hard coral is? That would rule out any corallimorph or sponge. But I don't think this is
Turbinaria. While shaped like the laminar "cup" morph of
Turbinaria Peltata, the polyp structure, the bladelike septa, the tentacle shape, and the presence of a central "mouth" seems to indicate otherwise. My educated guess is a species of
Fungia or similar plate coral species (
Heliofungia or
Halomitra). My only problem is that the baliktad shape! I've never seen a Fungid coral that is concave (shaped like a cup), rather than convex (although nothing says that it can't be like that). Is it still alive? If it survives, I would be interested to know about your success in keeping it, especially its feeding habits.
Hope that helped....
Phil