It looks bad at first but you want them to grow. Good rule of thumb is 4 to 5 inches apart. For vertical figuring, I always look at the type of coral.
Mille's IMO don't grow very tall. They grow up and then spread out so you could put something above it not too far away.
Torts generally just get BIG. They can get VERY tall quickly at least for me. I'd leave space above or trim and sell/trade.
Tables do not get tall at all and just spread out wide. I'd put them higher up and possibly put LPS or zoos or low light corals under them. Or have some acros trimmed to grow around it. (Looks nice).
That's just a few. A good reefer will realize how his corals are growing and move them to a better spot where they can grow. I don't personally care for letting them battle it out. If that is the case, clip it an inch back and send me a frag(unlesss it is brown). I'm about to snap my tort in half as it has doubled in vertical length and is wide as heck from a frag 7 months ago.