What does 'M' mean in the 'Free or Slave' field in the 1850 US Federal Census Mortality Schedule?
It's probably an error.
The options for the 'Free or Slave' column were just 'Free' or Slave'. Often, the default was taken to be 'Free', so the values was only entered for slaves with an 'S' in that column.
However, the column to the left of the 'Free or Slave' column is 'Color' with options White (W), Black (B), or Mulato (M).
On the other side, the column to the right of the 'Free or Slave' column is 'Married or Widowed', with options 'M' for married and 'W' for widowed.
Sometimes people entered the answer in the wrong column, which in this case could end up with an 'M' for 'Mulato', or an 'M' for 'Married' in the 'Free or Slave' column.
In the snippet from an 1850 census page below, the person completing the form entered the 'M' for 'married' in the wrong column on the fourth row.
In this case, someone spotted the mistake, crossed it out, and entered the 'M' in the correct column, but that didn't always happen.
Census takers were human too.
I believe that sempaiscuba is probably correct with their answer. However, there is a slim possibility that the "M" stands for "manumitted", which is the technical term for the act of a master freeing a slave ("the slave was manumitted" = "the slave was freed by his master"). It seems unlikely that the census taker would use an additional letter instead of just "F" or "S", but I suppose it is possible that they might have been trying to make a distinction between those who were born free and those who were born slaves and then later freed.
A simple transcription error is definitely the more likely scenario, however.
Out of curiosity, is the person in question listed as white, black, or mulatto? Because if they are listed as white, it is definitely a transcription error.