FATAL: password authentication failed for user "postgres"

It might be came from malicious traffic, check if the logs repeated almost each seconds like below

2019-04-17 16:10:39.228 UTC [32] FATAL:  
        password authentication failed for user "postgres"
2019-04-17 16:10:39.228 UTC [32] DETAIL:  
        Connection matched pg_hba.conf line 95: "host all all all md5"
2019-04-17 16:10:41.916 UTC [33] FATAL:  
        password authentication failed for user "postgres"
2019-04-17 16:10:41.916 UTC [33] DETAIL:  
        Connection matched pg_hba.conf line 95: "host all all all md5"
...
...

Lets verify TCP Traffic on port 5432 using tcpdump command

$ sudo tcpdump port 5432 and '(tcp-syn|tcp-ack)!=0'
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens4, link-type EN10MB (Ethernet), capture size 262144 bytes
16:19:26.472838 IP XX-XXX-XXX-XXX-static.xxx.xxxxxx.net.39903 > xxx.xxx.xxx: Flags [S], seq 1779566115, win 29200, options [mss 1380,sackOK,TS val 451453976 ecr 0,nop,wscale 7], length 0
16:19:26.473135 IP xxx.xxx.xxx > XX-XXX-XXX-XXX-static.xxx.xxxxxx.net.39903: Flags [S.], seq 4028889669, ack 1779566116, win 28960, options [mss 1460,sackOK,TS val 2412144 ecr 451453976,nop,wscale 7], length 0
16:19:26.525816 IP XX-XXX-XXX-XXX-static.xxx.xxxxxx.net.39903 > xxx.xxx.xxx: Flags [.], ack 1, win 229, options [nop,nop,TS val 451453989 ecr 2412144], length 0
16:19:26.525875 IP xxx.xxx.xxx > XX-XXX-XXX-XXX-static.xxx.xxxxxx.net: Flags [P.], seq 1:42, ack 1, win 229, options [nop,nop,TS val 451453989 ecr 2412144], length 41
...
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If this is the case then set firewall rules and necessary securities to combat bots or unwanted traffics.

in your pg_hba.conf

# IPv4 local connections:
# TYPE  DATABASE    USER        CIDR-ADDRESS          METHOD
host    all         all         127.0.0.1/32         trust

if it does not work then try with

host    all         all         your_ip/32         trust

then restart your data base it will work fine

If the username and password are correct then md5 is the correct value. Make sure you restart the database process or at least have it reload the configuration after modifying pg_hba.conf.