I heard that there is a whale that moved 8k BTC to an exchange and that what caused the price to go down in addition of Stop-Losses being triggered.Is that true?
The act of moving a large number of bitcoins to an exchange won't affect the price any more than moving them anywhere else. However, some people may panic and sell because they believe that the "whale" is about to sell the entire amount, and that can drive the price down.
If it's true I consider it to be a big problem as we clearly see that whales manipulate the market easily which is harmful.
My understanding of manipulation is that it is the attempt to change the outcome of a
future trade. As such, a "whale" simply buying or selling large numbers of bitcoins is not manipulation, regardless of the effect on the price.
Every trade, no matter how large or small, has an effect on the price.
Manipulation can be done by spreading false or misleading information, or also by affecting the price in one market in order to affect the outcome of a trade in another market.