dBm is dB relative to 1 milliwatt. 13 dBm is 20 mw.
dBm/Hz Is a measure of power density in the frequency spectrum, usually used to describe the noise power density across the frequency spectrum. It is the power density compared to a density of 1 mw for every 1 Hz of bandwidth. The power in uncorrelated signals (e.g. noise) at different frequencies can be added up in this way, and is proportional to the bandwidth. (It is related to dBmicrovolts per root Hz. If microvolts per root Hz are squared and divided by the relevant impedance the result is in pw/Hz).
-171 dBm/Hz is not in mw. You need a bandwidth to convert it to mw.
Use your calculator and the instructions in the posts above or the rule mw = 10^(dBmw/10) to calculate mw/Hz from dBmw/Hz. Unless you have a bandwidth (mHz, Hz, kHz, MHz, GHz) you cannot get milliwatts from dBm/Hz. It is like asking how many miles is 20 miles an hour. All systems actually have bandwidth.