This can happen in some cases because the watch was put together in a humid environment, so it is said.
The above may be so but I wouldn't like it. My reaction would be to send it back for a refund or replacement. Note, refund or replacement, not repair.
However, if getting a refund or replacement is going to be difficult (depending on where you bought it) I'd check to see if the Casio warranty is valid. I don't know offhand if Casio warranties are worldwide, require dealer stamps, etc., so I'd check on that. If the Casio warranty is valid then I'd do nothing -- just carry on wearing the watch! If the watch actually goes wrong then send it back to Casio for a warranty fix.
If you establish that the Casio warranty is not valid and, as above, getting a refund or replacement is going to be major hassle, then a bit of DIY: Open up the case, leave it somewhere warm and dry for a bit, close up and see what happens. Perhaps only do this if the condensation reappears.
Well, that's what I'd do.
N.B. It is normal for there to be some moisture in any and all watches, unless they are sealed in a protective atmosphere or have some sort of desiccant inside. So some condensation does not necessarily indicate likely failure.