To expand on my statement above a bit...
Each person in my family of 3 is particularly adept at spotting some things and particularly poor at others. I appear to have some sort of barracuda blindness, for example. We dive with a former classmate who finds the most minuscule things. She'll point to an anemone or sponge and we'll see... nothing. We'd joke about her hallucinating things. Then I started taking pictures of the "nothing" to prove there was nothing to see. Back on land, Tamara would say "See, there's that tiny shrimp" and I STILL wouldn't see anything, even with her pointing right at it. However, zoom in onscreen and I'll be damned; there's a little shrimp or whatever, just as she'd claimed.
Divemasters have a number of advantages. Not least is knowing where on a given site lionfish (or seahorses or whatever they're seeking) like to hang out and where they've been found before. They also are so much more familiar with each site than even frequent recreational divers, so are more equipped to notice anything that's different, including very subtle things such as shadows or movement. Finally, they're intimately familiar with what indirect signs to look for. For example, the angelfish that crowd around feeding sea turtles snatching up sponge fragments can be easier to spot from a distance than the turtles themselves.
When I was teaching medicine and asking students to observe patients, they'd start out the rotation convinced I was making stuff up or knew the answers beforehand. The tremor was too subtle, the rash too pale for anyone to notice without cheating. By the end of the rotation, they were much better at picking up those things themselves. No magic glasses required.