I personally like to think that at first Rocky doesn't understand human idiomatic phrases, slang and metaphors, and takes them too literally but once he realizes that Grace uses indirect language often he picks up and adapts really quickly to it because Eridians have similar figurative language.
The difference between their languages though is that Rocky doesn't understand that a lot of those indirect phrases that humans use have been well established for decades if not generations and have an inherent meaning while Eridians tend to come up with them on the fly to suit their needs so his attempts to replicate similar patterns of old sayings are nearly indecipherable to Grace.
Grace one time had to explain what "piece of cake" meant and now Rocky regularly says weird shit like "That was a carapace brush." Or "Optimal amount of dirt on my eggs." And Grace is supposed to extrapolate meaning from a phrase that has never been uttered by either species before every fucking time.
