If you've seen a coral that looks like a bunch of tiny hammers swaying in the water, that's a hammer coral, or Euphyllia ancora. It's an LPS coral, which means it has a hard skeleton under all that soft, fleshy tissue, and it's one of the most eye-catching corals you can put in a tank.
The good news is it's not hard to keep once your tank is stable. Hammers want medium light, not blasting right under your LEDs. Middle or lower in the tank usually works. Give them gentle, moving flow so the tentacles sway, but don't point a powerhead right at them, because they'll pull in and sulk.
The one thing to watch is space. Hammers send out long sweeper tentacles at night that can sting anything nearby, so give them a few inches of room from other corals. Other Euphyllia like frogspawn usually get along with them fine.
They get most of their food from light, but you can spot feed small meaty stuff now and then to help them grow. Keep your alkalinity steady, because big swings are what usually make hammers go downhill.
Quick tip: if yours is fully puffed out during the day, it's happy. If it stays shrunk up for days, check your water first.