I'm trying to imagine a super scary and jealous Yuta, but atm he's too cute. I don't think Ran is walking on ice, Ran's vision of Leez is a baby chick and even admitted that he wasn't the slightest interested in her when she got hit by that Yama spell.
I think the reason for Yuta's reaction is because those two are acting childish and Ran's supposed to be so much older than Leez.
The reason why Yuta's probably okay with Alexander, is because he will protect Leez. You saw how he acted with Sagara, he doesn't care about her, the only thing that matters is that Leez got hurt because of him/ her. Alexander saved Leez from getting beaten to death, so Yuta likes Alexander more than Sagara or any other sura who hurts Leez, even if he's his love rival. Leez well being comes first. The jealousy can come when he reached 3rd stage and is sure that Leez doesn't hate sura anymore.
Yuta is also very aware at how dense Leez is when it comes to romance. She's young and naive and never has a guy showed any romantic feelings towards her (in her point of view). Let's see how Leez for react, if Yuta or Alex do something very unexpected, like giving her flowers or complimenting her on her looks, even thought others call her ugly.