Impurity can get inside from HCl, bleach or foils. There is no other feed into the reaction.
HCl is often fairly contamination free, except some dissolved iron (yellowish discoloration of technical stuff).
Bleach, I don´t know. It is made by electrolysis of NaCl solution or by passing stream of chlorine through solution of NaOH. There is not very much possible impurities from the production process, aside of minor metal contamination, if it was electrolyzed. Problem is, that Clorox or any other brand could add some stabilizers or other chemicals to it. This could pose a problem. And impurities are completely unknown.
Third possibility is contamination from foils. Lacquer, some polymer junk coating etc is one possibility. Some low solubility metal precipitate is the other. Tin compounds form fluffy voluminous oxochlorides when tin (II) in solution meet oxidizer. It could also aglomerate to form some better packed precipitate. From the photo it does not quite look as Sn(IV)OxClx stuff...
XRF would be helpful in way to identify the impurity, but I am maybe "over-analytical"
filter and proceed further.
Just to note, bleach is quite ineffective oxidant for dissolution of gold. Chlorine escape from the reaction vessel quite quickly - so you will need to add quite a bit of bleach, which itself is also very diluted (5-13% - and never trust the label, it´s always less
bleach decompose slowly over time). If you want to improve the efficiency of chlorine adsorption, take thin tubing (like for medical use) and dose the bleach slowly under the level of solution (best from the bottom). Use taller beaker, so the column of liquid escaping gas need to pass is greater. And do not overshoot with temperature.
It might take longer to dissolve at room temperature, but you use less bleach to accomplish the dissolving.
Also cover the beaker with some plastic wrap or watchglass, round bottomed flask etc. to eliminate evaporation loss of chlorine.
If it is possible for you, purchase what is called here "shock pool chlorine" which is practically granulated calcium hypochlorite (check the label). It is relatively cheap (i managed to get 700g for like 6 EUR in local supermarket pool section) and quite soluble in water. You can make as strong as 20% solution, and best is it does not decompose over time nearly as bad as aqueous bleach.
Be careful, one good accidental whiff of chlorine and you could end practically disabled for life. Do not take any chances
Stay safe
I wish sucessful refining