Serpias flirting with Sapnap in front of George because he knows what people have said, he's heard the rumours and seen the clips. He knows that George and Sapnap have some sort of thing going on, but he wants Sapnap for himself.
He's not going to be a dick to George or anything - not directly, anyway - he's just going to flirt with Sapnap in a much more efficient way, and he's going to be kind of obvious about it. He's got to make up for lost time, after all. He's competing with a guy who's been friends with Sapnap for years.
Every chance he gets, Serpias is touching Sapnap in some way, purring some compliment in his ear, pointedly ignoring George (most of the time) while he absolutely showers Sapnap in positive attention.
George's way of getting Sapnap's attention is to get him riled up, to try pissing him off. Serpias prefers to flirt and compliment and appear much more likeable.
The language barrier makes things a bit harder, but even when he says something to Sapnap in Spanish, he can spot the blush dusting his cheeks. He knows he's got the upper-hand, despite his disadvantages.
And if Serpias likes the glares he gets from George, feels more fueled by the Brit's scoffs and eyerolls, then so what? If George really wants Sapnap so bad, then he should just do something about it.