Oh dear. Rapid heavy breathing and u say it's scratching itself is not a good sign. Can't comment about mucus in mouth.
If it's gone from acting normal to this behaviour very suddenly and your really worried u can do 2 things....
1. Take it back to shop and ask 4 refund.,say it is showing signs of possible gill flukes
2. Keep it and see what happens..u can add salt and up temp to 88-90. Remember after
A Tiger has eaten will will seem like heavy breathing, where they never close there mouth properly for several hours ....whilst they are digesting food..also some new tigers will lay on side in corner of tank when In new environment.