mining difficulty and hash rate - should I care about it as a trader?
if I should care about it, can you please explain why? how and both of them affect the cryptocurrencies price?
No, you needn't worry about it. The only (outlandish) scenario is when the hash rate starts dropping so low that the probability of a 51% attack becomes non-negligible. Then the price might crash. Given how the hashing power and difficulty keeps increasing, you needn't worry about that happening now.