Your slave is sending an array of characters and your master is reading the ASCII values of those characters.
If you send 25.43, Serial.println will send 7 characters 2, 5, ., 4, 3, carriage return and a newline.
The master will read them as 50, 53, 46, 52, 51, 13 and 10.
On your master, you should replace
tempC = Serial.read();
with
tempC = Serial.parseFloat();
Serial.read(); // remove carriage return character from the serial buffer
Serial.read(); // remove newline character from the serial buffer