To calculate NEP in W/Hz^1/2 first calculate the total noise power within your measurement bandwidth. (When working with noise, power is the more fundamental quantity you want to use, rather than A or V.) To do this, get the noise power spectrum in units of A²/Hz. In your case, square the current spectrum you are showing and divide by the bandwidth of a single spectral bin. Depending on how you got your spectrum this may simply be equal to the frequency step along your frequency axis, or it may be more complicated if you used windowing functions or measured it directly with a spectrum analyzer.

The total noise power, in A², in your measurement is the integral of this spectrum over your bandwidth. (I believe you can also get here from the standard deviation of the noise timeseries measurement. This would automatically account for your measurement bandwidth. There may be an additional conversion I'm forgetting though.)

To get NEP in A²/Hz you take your in-band noise power and divide by the bandwidth you integrated over. Take the square root of this to get A/Hz^1/2 and apply your responsivity to get to the desired NEP in W/Hz^1/2 for your bandwidth.

Its possible to do the calculation in fewer steps if you work out the math, but I think this breakdown makes it more clear what is going on.

To calculate NEP in W/Hz^1/2 first calculate the total noise power within your measurement bandwidth. (When working with noise, power is the more fundamental quantity you want to use, rather than A or V.) To do this, get the noise power spectrum in units of A²/Hz. In your case, square the current spectrum you are showing and divide by the bandwidth of a single spectral bin. Depending on how you got your spectrum this may simply be equal to the frequency step along your frequency axis, or it may be more complicated if you used windowing functions or measured it directly with a spectrum analyzer.

The total noise power, in A², in your measurement is the integral of this spectrum over your bandwidth. (You can also get here from the standard deviation of the noise timeseries measurement if it is collected at the same frequency/bandwidth at which you want to calculate NEP. Doing so would automatically account for your measurement bandwidth. There may be an additional conversion I'm forgetting though.)

To get NEP in A²/Hz you take your in-band noise power and divide by the bandwidth you integrated over. Take the square root of this to get A/Hz^1/2 and apply your responsivity to get to the desired NEP in W/Hz^1/2 for your bandwidth.

Its possible to do the calculation in fewer steps if you work out the math, but I think this breakdown makes it more clear what is going on.