It is likely that this is polymerised oil. The oil reacts with oxygen to polymerise. Heat and the presence of metals may accelerate this polymerisation. It may take on a colour depending on the metal (eg Iron in your case may be the cause of the reddish colour).
I am pretty sure that this is the case here.
If it is causing you a problem with rapid filter blocking, then when convenient, pull the fuel tank out and have it cleaned to remove any build-up in the tank, rinsed out, then dried before reinstalling it. This should give you a few more years before it becomes an issue again.
Others may have other suggestions on how to deal with this issue.