Hey all, just got these two lovely baby clarkiis from LA. I noticed that both have a grey/blackish spot on the tail. It's more pronounced on the one on the left than the one on the right, but they definitely both have it. Is this just a marking, or is this something that I should be worried about? please see their tails in the video below:

thanks in advance for your advice!
Matt
thanks in advance for your advice!
Matt