Oh, more grass and debris.
For now, you hold your fire, uncertain what you should do; you glance back at Sergey, a sudden feeling of dread overcoming you that you've walked into an ambush. All you see, however, is Sergey patiently lining the thing up in his sights.
"What are we doing?" you hiss - if this were a straight up gunbattle, you'd be leading the way, but this is something new. Sergey's answer is equally curt.
By and by, you slowly move into a line - you don't want Sergey behind you, and the others seem to get your suspicion.
The mutant, whatever it is, dives under water again, only reemerge and repeat the gasping ritual. Now it breaks from routine, stepping onto land and casting a baleful, dead glance over where you five are in cover.
It stops. Makes that gasp-gargle again.
And bursts into loping, hopping charge. You feel a dreadful spike of fear hammer into your head as the thing's black, featureless eyes roll across you.
Roll 2d20. One perception, one attack on the thing.