At face-value, those words have seemingly equal attestation or support, but actually? From 5 years study of (and relatively little progress in) Latvian, I know that we can expect that every 3 - 5 year old Latvian speaking child comprehends and uses slapjš. It is core childhood vocabulary word generally describing the state of things affected by water. Perhaps the meaning is narrower in Lithuanian or perhaps not, but I myself wouldn't trust what I might say about it. κλέπας is given in LSJ, in what I assume is a verbatim quote from Hysechius:Page 436 wrote:Lith. šlapias, Latv. slapjš 'wet' : - : : Gk. klépas 'wet'
While one might be tempted to ammend δασύ to a form of δροσόεις when the apparent mis-match in the 'wet' set is noticed, another (perhaps more preferable) solution is possible too. The LSJ entry for δασύς contains reference to a later writer:κλέπας: νοτερόν, πηλῶδες: ἢ δασύ, ἢ ὑγρόν, Hsch.
i.e.I.3 wrote:3. generally, rough, thick, “μαλακαὶ καὶ δ. νεφέλαι” D.S. 3.45.
The main idea of the passage is that rain from moist clouds makes a difference. Presumably, the type of clouds described by DS are nimbostratus clouds - with their prolonged periods of rain.Diodorus Siculus, Bibliotheca Historica, 3.45.6 (part),7 wrote:ἡ δ᾽ ἑξῆς χώρα κατοικεῖται μὲν ὑπὸ Ἀράβων Ἀλιλαίων καὶ Γασανδῶν, οὐκ ἔμπυρος οὖσα καθάπερ αἱ πλησίον, ἀλλὰ μαλακαῖς καὶ δασείαις νεφέλαις πολλάκις κατεχομένη: ἐκ δὲ τούτων ὑετοὶ γίνονται καὶ χειμῶνες εὔκαιροι καὶ ποιοῦντες τὴν θερινὴν ὥραν εὔκρατον. ἥ τε χώρα πάμφορός ἐστι καὶ διάφορος κατὰ τὴν ἀρετήν, οὐ μέντοι τυγχάνει τῆς ἐνδεχομένης ἐπιμελείας διὰ τὴν τῶν λαῶν ἀπειρίαν.  τὸν δὲ χρυσὸν εὑρίσκοντες ἐν τοῖς φυσικοῖς ὑπονόμοις τῆς γῆς συνάγουσι πολύν, οὐ τὸν ἐκ τοῦ ψήγματος συντηκόμενον, ἀλλὰ τὸν αὐτοφυῆ καὶ καλούμενον ἀπὸ τοῦ συμβεβηκότος ἄπυρον. κατὰ δὲ τὸ μέγεθος ἐλάχιστος μὲν εὑρίσκεται παραπλήσιος πυρῆνι, μέγιστος δὲ οὐ πολὺ λειπόμενος βασιλικοῦ καρύου.
The next tract of land that I'd like to consider - that ihabited by two Arab tribes, the A. and the G. - is not scorched like the adjacent lands, but it is quite frequently filled throughout by soft and moist clouds. These produce periods of rain and seasonable winter storms, resulting in a mild climate during summer. [Goes on to talk about the beneficial effects of the rain]
Not mentioned in the LSJ entry for δασύς, but suggested by association in the word list given by Hysechus for κλέπας, and plausible in Diodorus Siculus, the meaning 'wet' in a broad range of senses is similar to the range of meanings that the Latvian slapjš is the meaning 'wet'.
We usually think of Hesychus as the authour of lists of obscure words, but here is a word with quite a range of meanings; νοτερόν (the wet or moist whatever that is itself, but now the wet version), πηλῶδες the muddy (ground, path or field): ἢ δασύ (the water-ladden clouds - if my analysis of the DS passage is sound), ἢ ὑγρόν (the moist whatever that changed from solid or gaseous to "liquid" by the addition of water). Rain-bearing clouds are typically not as translucent (sunshine blocked) as non rain-bearing clouds, because are they are "thick", and that "thickness" comes about they are laden with (condensed) rainwater ready to fall.
If we do not explain away κλέπας (with its neuter adjectival ending -ας) as the record in Greek script - using kappa to represent a palatalised silibant - of an exotic word akin to Lithuanian šlapias, brought by a wandering (or enslaved) Balt, then we are left with the problem of explaining an "obscure" Greek word, with a very broad range of meanings. I think Hesychius or one of his predecessors may have had to have seen it in at least 3 or 4 different contexts to make that entry.