Decline in current during electrolysis
Given the whitening observed at the cathode, one potential explanation is that calcium salts were precipitating there, forming a layer blocking passage of part of the current.
I usually write the cathodic hydrogen-evolution reaction in neutral solution as:
$$
\ce{2 H2O + 2 e- -&gt; H2 + 2OH-}
$$
Thus, the pH in the vicinity of the cathode will be considerably greater than 7 during the electrolysis process, due to the $\ce{OH-}$ formed. Since you're using a calcium-based electrolyte, it seems likely that some combination of $\ce{CaO}$/$\ce{Ca(OH)2}$ is precipitating in that alkaline surface layer. Once formed, in my experience such solids will dissolve only slowly in neutral aqueous solution.
If possible, I'd recommend trying the experiment again but in a different electrolyte, such as $\ce{KNO3}$ or $\ce{NaNO3}$; those alkali cations shouldn't exhibit any precipitation.