Version of Moodle could use a bump up ... to 3.5.8 ... that version is a long term support version .... so bump it up. It is still supported for security fixes, BTW.
Ubuntu 18.04 ... who does OS updates? Is this a managed server ... where provider does heavy lifting ... or un-managed server ... where you do the heavy lifting. If latter, when was the last time you did an OS update?
You sure your IP is that? If so, looks like you need to admin some more. Going to that IP via browser begets the default welcome page that says:
"This is the default welcome page used to test the correct
operation of the Apache2 server after installation on Ubuntu systems.
It is based on the equivalent page on Debian, from which the Ubuntu Apache
packaging is derived.
If you can read this page, it means that the Apache HTTP server installed at
this site is working properly. You should replace this file (located at
/var/www/html/index.html) before continuing to operate your HTTP server."
Are you allowing guest access? to Parents?
You could do some sluething by looking at your web servers access logs to see if there is anything that looks out of the ordinary ... of course, that would mean you would have to be familiar. ;)
Hosted with? Not all providers are equal when it comes to security of systems leased by customers.
** Example **
Moodle code through the years has been really good security wise ... if kept up to date.
If what's been reported 'in the wild' seems to me there would be many in this forum saying 'me too'.
Can't give a pat ... 100% correct answer here in forums ...not sure anyone here in community could.
Like I said ... don't panic! After a little more admin and checking for updates to OS ... sluething access logs ... plus any logs you have ... like secure log ... looking for un-authorized access, etc. Put on a pot of coffee and dig in.
'spirit of sharing', Ken