The forward slash is not supported in part of the filename for saveaslayerfile tool. In fact, I can't even name a dataset with a slash in it. That must have been added in the TOC of the MXD itself?In any case, it appears as if you are getting the list of layers from the current mxd, so suspect you have no control over the name in the TOC (would be easiest just to not have the slash in the name), so.Could do something like this that would replace any forward slashes with an underscore (this is what ArcMap does if you right-click on that same layer in the MXD and select save as layer file).
# Change Feature Classes to Relative Path Lyr files
import arcpy, csv, os
from arcpy import env
env.overwriteOutput = True
mxd = arcpy.mapping.MapDocument("CURRENT")
messageStr = str(mxd)
arcpy.AddMessage(messageStr)
arcpy.env.workspace = "C:\\GeneralWorkData\\ArcGIS Test Data\\Test Data From,to Martin B\\Relative_Paths_Lyr_Files"
for fc in arcpy.mapping.ListLayers(mxd):
fc = str(fc)
out_layer = fc.replace("\\","_") + ".lyr"
arcpy.SaveToLayerFile_management(fc,out_layer,"RELATIVE")
messageStr = out_layer
arcpy.AddMessage(messageStr)
R_If this doesn't work, then perhaps you are running into this issue (from the help docs) that will also throw the 000732 error: An invalid subtype on the dataset. To fix this, go to the feature class properties, then click the Subtypes tab and reenter the default subtype code. If the default is 0, click the cell with 0 and reenter that same value. Then apply the change by clicking OK. You will now be able to use the dataset.