This looks like hemorrhagic septicemia.
Test your water. If you do not have a test kit / know your water parameters for ammonia, nitrIte and nitrAte then just do a couple 50% pwc. One right after the other.
See if that fish or the other fish, looks any better.
If you do not have a cycled tank then I would do one more 50% pwc. soon thereafter.
Getting a liquid test kit asap.
Also you can simply change the water with treated (declorinated) water which is temperature matched daily like 50% per day.
Having a test kit is VERY important. Otherwise you will not know if the fish are living in safe conditions or not.
What size tank?
How long has it been running?
What type of filter?
Are you familiar with the nitrification cycle?
Temperature?
Read this after doing the water changes.
https://www.aquariumadvice.com/guide-to-starting-a-freshwater-aquarium/