Which matters does this refer to?
Rashi: Binyan ha'Mikdash.
Who is Artachshasta?
Rashi (based on Rosh Hashanah 3b): It is Daryavesh. Ibn Ezra - all Malchei Paras are called Artachshasta; this was Daryavesh. Malbim - according to this, Ezra ascended a year after Binyan ha'Bayis.
Malbim according to Ba'al ha'Ma'or, Redak (Sof Chagai): It is a king who ruled after Daryavesh. If so, Ezra ascended long after Binyan ha'Bayis. There are many opinions; one cannot decide [which is correct].
Was Ezra Serayah's son?
Malbim: No, he was his grandson, like it says in Divrei ha'Yamim 1 . Six people in the chain of lineage listed in Divrei ha'Yamim are omitted here.
I did not find so explicitly there, but Rashi (Divrei ha'Yamim I, 5:41) says that Ezra was the son of Yehotzadak ben Serayah. (PF)
