Predict Ethereum price up or down by given Bitcoin price
So, the question asks how to predict the price Etherium given the price of Bitcoin.
It is worth doing some Explanatory Data Analysis (EDA) to see if there is a correlation between these two variables.
You might find that the relationship between the two variables is non-linear, which indicates that maybe the model that you would make with Bitcoin price feature is going to be too simple and will lead to undercutting. Therefore, it would be worth adding more complexity by increasing the number of features, which could also be correlated with Etherium prices.