No, that is overfull and the 'stuff' is being forced out by stuff
backed up the inlet pipe.
We had the same problem.
We popped the cap, and "stuff" flowed out, because the leach field had been
overwatered, and we probably had put too much washing machine water down
there during that time. We found out about it by the pipes backing up into
the house (minimally, luckily). So, you can have a backup into the house
for two reasons: a plugged main drain, or a full tank that isn't going into
the leach field for ANY reason.
We had it pumped, stopped watering the grass on the leach field, stopped
washing as many loads of clothes, and watched the performance of the tank by
leaving the plug accessible, and checking until it got to the leach field
overflow, then watching that it was running out into the leach field, which
it did, so our problem was overwatering and washing too many loads. We do
not have a rain problem or water infiltration problem as some people in some
other areas and climates might have. We artificially induced that by twice
a day watering of the lawn.
The solution is to find out what is causing the effluent NOT to flow out the
tank and into the leach lines. Or find out if you have a clog in the main
pipe. Some companies have TV cameras and all kinds of spendy stuff. Our
guy is one of the honest ones, and he said to quit watering, let the tank
fill up, keep an eye on it, and if it ever backed up, he'd send out the
leach field guy and he'd diagnose what was wrong with it. Most companies
want to sell you the full Monty, camera inspection and all. Once that is
established, by expensive or cheap common sense measures, action can be
taken up to but not including adding to or replacing the leach field. BIG
Steve, who still has his fingers crossed.