Why is the Depot Storage of RSS showing the red arrow going downward and the rss dropping?

For Food and Lumber there will be a rate of consumption by your troops. The more troops you have the higher the consumption rate. If this exceeds your current production rate then your amount of stored resource (in the warehouses) will go down.

There is a safeguard though. When the amount you have gets below a fixed value the consumption will stop so you won't lose any more. When it is below this point your amount will increase until it reaches that value. As you upgrade your castle the minimum value will increase.