Sub with GEC on Neutral bar

Sub panel fed with a 2/0 copper feeder on a 100 amp breaker through metal conduit with appropriate bonding bushing. Neutral bar floating, No EGC’s since armored cables. There is a wire with green tape over red sheathing (#8 - I believe) signifying its designated as a ground. One ends goes out to a ground rod i assume since it goes through the wall to the ground where i can not physically check for rod, the other end is connected to the neutral bus bar with all the neutral circuit wiring ( white) . I believe this is wrong or is there something with metal conduit that allows this for some reason.

So this feeder is for a separate structure? If so, that structure would require a grounding electrode system (GES) which can be one or two ground rods depending on the code cycle that it was installed under. The conductor going to the rods is a grounding electrode conductor (GEC) and can be any color, no green tape required. Also the GEC is sized according to the #2/0 feeder conductors and NEC table 250.66 which is a #4 copper conductor. Since it’s a ground rod it’s not required to be larger than #6.

The GEC must not be connnected to the neutral bus, it should be connected to the ground bus if there is one or directly to the metallic enclosure. Bonding bushing is not required.

Not a separate structure, should’ve mentioned that prior. Im assuming the metal conduit (over head service drop to meter then to service disconnect switch then to sub panel 10 feet away) is the GEC being grounded at the meter ? Would this be correct?
Panel is filled with tandem breakers - not allowed by panel label, and 2/0 copper is in double pole main rated up to 1/0 only in the sub. Definitely referring electrician but that ground wire on the neutral bus threw me off as knowing it was wrong but maybe with some situations, it may be OK. Obviously not.

Btw. There was No ground bus bar , only a neutral. There were no EGCs from the circuit wiring, in armored cable mostly although a few circuits came from a 1-1/2 inch metal conduit , which i assume was NM circuit wiring. Couldnt see any sheathing. Why there are no EGCs from these circuits is confusing. Majority of outlets tested pos. for grounds. Hopefully not bootlegged as i review it now

The GEC(s) must terminate in the service disconnect not the subpanel.