When Lex sent a speck of energy inside Pearl's body, his actual goal was not to directly increase her realm. He could do that, of course. The energy he produced was the purest, meaning it could easily adapt to any form and seamlessly become a part of her cultivation.
But doing such a thing was only a short term benefit. Why would he do something like that when he could, for the same amount of energy, provide her with both immediate and long term benefits, creating a sturdier foundation upon which her cultivation would be built?
They were inside the house assigned to River, so there was no concern about getting spotted or having their actions discovered. Of course, if the elves were secretly spying on them River and Pearl would be unable to detect it, but Lex would. Fortunately, the elves had tried nothing of the sort. Even if they did, they would not discover anything if Lex wanted to keep things hidden.
