Is this ok ? ( video)

I bought my rays yesterday and was having a good look today . The female seems to have a bit of a *worn* edge to it's what has been refered to as feet ,
in the YouTube vid you can see the fluffy edge . Am I just paranoid being a newbie keeper, or should I take it back to the shop,

Vid link
http://www.youtube.com/watch?v=--Cf2Cem3TI&feature=youtube_gdata

I test my water daily ammonia 0 nitite 0 nitrate 15 . I change partial water daily and am very fussy about my water parameters . The ray seems pretty active and I am keeping the light down for a week or so . Keeping a pair of rays is something I am very keen to do so I may just be worrying too much after Reading up that it's not an easy job .
Thankyou for any replies on this . It puts my mind at rest to know that other owners are willing to donate some time to helping a newbie ray keeper

Cheers bud . I looked again today to find the fluffy layer has thickened slightly . I called the store where I got her from and they are letting me return it for treatment tomorrow .
The male is fine and eating well so that gives me a bit of hope with keeping stingrays . Will keep you posted and will try and get a good pic when in the bag to show the affected area . I really hope it's something silly that a bit of salt etc will get rid of .

It was fine sand but have removed it all as a precaution for a while and will re introduce a bit when rays are settled

Update . She did not make it ( sigh)
so a mark on the fin turned to death in three days . It unbelievable how sensitive these guys are. The shop are great and offered another one but I am going to wait to make sure the other one is settled for a couple of days . Then I will collect the replacement .

The tank is now bare bottom and my male is eating up bloodworm as I type so just hope nothing develops before I collect the replacement female .
I cannot see any reason to worry about my other one . All seems good there