Q; How come taking the life-form away from its environment didn't kill it?
A; We might assume that taking the life-form away from its saltwater environment would indeed kill it but not straight away. Indeed, not only did the specimen aboard the Enterprise survive for the duration of the show, it actually managed to prosper, if we can interpret reproduction as such. It only started to fade when the lab lights were turned down. My opinion is that the lack of both the saltwater environment and light were too much for it, which is why it cried 'uncle'.
Q; If these creatures need light to survive, what do they do at night?
A; We might assume that the life-forms survived the planetary nightfall the same way much of our wildlife survives the winter. Simply, they migrate! The main difference would be that while our wildfowl migrate on a seasonal basis, the life-forms migrate on a daily basis. Or, maybe they stay up and watch telly.
As has been observed elsewhere (specifically The Nitpickers' Guide), the viewscreen magnification in Gates McFadden's lab close-ups were wildly out of sync. Maybe that's what happens when ugly bags of mostly water do things in too much of a hurry!
The patterning in the sand made by the life-forms does make you wonder about crop circles, though, eh?
I noticed that when Beverly asked for Picard to come to Sickbay from the Bridge, he responded without a word. Where does this leave Beverly? If he doesn't respond verbally, how does she know what's going on?