The Welsh side also beat Sheffield Wednesday 2-0 in their FA Cup fifth round replay on Tuesday to progress to the last eight for the first time since 1964.
"They left several players out of their team, but they have improved and they have gained confidence," Moyes told a news conference on Friday.
"They had some incredible results against Arsenal and Liverpool, picking up unexpected points. We're in for a difficult game.
"It is a tough game going there but we have had some good results away from home, hopefully we can continue that."
The Scot was also concerned about his team's journey to the Liberty Stadium in south Wales because of heavy snow and freezing weather conditions.
"The Met Office is saying people shouldn't travel, especially through Cardiff and down the M4," Moyes added.
"It is a concern at the moment and I'm guessing it shouldn't be any different for football teams and fans. I think the Premier League may need to look at that."
Moyes also said that West Ham's chances of survival will depend on the club's results in their five remaining home league games as a tightly-contested relegation battle heats up with 10 fixtures left in the campaign.
(Reporting by Aditi Prakash in Bengaluru; Editing by Keith Weir)
Share
