serial data gets spoiled
The b prefix in Python 3 just means that it is a bytes literal. It's not part of the output, that's just telling you the type.

The \r\n is a common Carriage-Return and Newline line-ending characters. You can remove that from your string by calling strip().

Since these are floating-point numbers being returned, I'm guessing you're going to want to use them in some way after they are read, too:

import serial

ser = serial.Serial('/dev/ttyACM0', 9600)
while True:
   value = float(ser.readline().strip())
   print('New value is {0:0.2f}'.format(value))


See also:


What does the 'b' character do in front of a string literal?