Secrets in Nioh are little-known mechanics or special items that have a double purpose, usage, or are cleverly hidden.
There is a way to get past Nurikabe without fighting them. Stand before them (so that they can see you) and do a gesture. You have two attempts to please the Nurikabe. Depending on the color of your gesture, the Nurikabe will greet you back and leave. Otherwise, he will grow angry, becoming hostile after the second failed attempt.
Sometimes (but not always), there may be some revenant graves near the Nurikabe if another player died to it or if one was placed by the developer. The grave's description will read "Crushed to death by Nurikabe for his hostile/neutral/friendly behavior" while being colored red, yellow, or blue respectively. This description indicates one of the two colors of gestures that angered the Nurikabe and led to that revenant's death, meaning that color gesture should not be performed. Since two attempts are given to please the Nurikabe, only one grave with this description is needed; simply perform both of the other possible color gestures with the two attempts and one of them should allow you to pass.
Unverified strategy: Start out by doing either a BLUE or RED gesture, and pay close attention to the Nurikabe's eyes. If your first attempt is incorrect and the color of the Nurikabe's eyes don't seem to change color that much, you should follow up with a NEUTRAL (YELLOW) gesture. However, if the eyes redden in color, you should use the OPPOSITE of your first choice, i.e. if you gestured BLUE first, you should follow up with a RED and vice versa. There have been reports of this working nearly all of the time. It may take a couple of tries to tell the difference in eye change, but should quickly become second nature.
Alternatively, if you can get behind them you can remove a spell tag and this will kill them, though it appears to sacrifice any Amrita or drops you may have gotten.
Mimics can be identified by the number of horizontal bands on a treasure chest. If a chest has two, it's safe. If there are three, however, that chest contains a mimic. You call out the mimic by using the WHISTLE gesture.
If after whistling you allow the mimic to come out of the chest, it will perform a gesture. Doing the same gesture will please the mimic, and it will leave all its loot behind without turning hostile. Note: there is a time limit in which you must perform the gesture, so move quickly.
Find and guide these little cute guys back to the shrine to unlock bonuses for the areas you are in, that you can activate at the local Shrine for a small fee.
You can converse with the blacksmiths and spend gold to unlock special perks that grant discounted prices, increased stock and even a barber shop
These special thermal waters give your player a temporary health regeneration effect. They can be used by multiple characters in the world (so your co-op partner can use them too), but only once per mission attempt. You will have to reload the level to use them again (pray at Shrines). Hot Springs negate all status effect, including helpful ones like Divination.