Much to look at here. Stephen has just been complaining about Haines and his nightmare. Mulligan is changing the topic, staying on his tear about "Hellenization."
In the second panel, Rob has drawn Mulligan and Stephen in an odd pose. Stephen seems to be surprised in mid-phrase, and Mulligan is reaching into his pocket. Specifically he "thrust his hand into Stephen's upper pocket." It's an interesting moment, one that the comic allows us to show the body language for. Mulligan is intruding, being forward, in Stephen's space. "Thalatta thalatta" means, unsurprisingly, "The sea, the sea!" It's from Xenophon. You can look it up...
A small textual point--there's an omission in this early draft--Mulligan says "Lend us a loan of your noserag to wipe my razor" --we left out the "your." Also, in the Rosenbach manuscript, Mulligan's first mention of the sea in this moment is "she is our "great" sweet mother." That's in Joyce's handwriting, and it's quite clear. It's repeated a few lines later. But in his errata for the first edition, Joyce specified that he wanted this to be "grey" sweet mother. A nice allusion to grey-eyed Athena, Odysseus' protector, but otherwise obscure.
And as for the Greek-- "Epi Oinopa Ponton" means (according to Gifford) "upon the winedark sea," a common epithet in Homer's Odyssey. This is another moment when I wonder if Joyce was raising another flag to his readers... "Hey! The Odyssey! It's important!" We know the Odyssey is important now, eighty years after it was published... but this might have been a more useful to early readers.