I'm not convinced this is a female GBR but
I'm fairly certain you're looking at fin damage rather than
Ichthyophthirius multifiliis or white spot disease - those fin abrasions are fairly common in newly imported rams or rams that are stressed/harassed
You might treat for white spot just in case, but I'd keep this fish rather than doing a return - how is her respiration rate? active? swimming? eating?
It sounds as if you're going back to the shop, so stop & look at their GBR tank, get comfortable & observe the fish for 15 min or so - if white spot you'll see much clearer signs than what was observed on your fish.
If Seachem ParaGuard is available, this is also fairly shrimp safe (any shrimp that have just moulted will be very sensitive to any changing water parameters for ~36h), I've also used Hikari Ich X "saltwater" with minimal shrimp losses (this is just a formalin based solution with no additional dyes)
I'd recommend doing an 70% water change right the way to take as much of the white spot spores out if the water
This is always sound advice when any disease is suspected
EXCEPT
care must be taken to check tank vs tap parameters - easiest is to use Tetra 5in1 or API 5in1 strip tests - or just stick with conservative 25% water changes, you can perform these every 12 - 24 hours; if any case of suspected disease, optimum water quality (ie very "clean" water) is the best treatment you can offer - if there's a raging infection, you're going to need medication as well.
(I completely disagree with the oft stated "test kits are useless in aquaria" - they are not automatically inaccurate etc, many of these kit methodologies are used successfully with soil & food samples ... I doubt that many tanks are "swampier" than that
Note for each kit methodology there is a list of known interfering compounds, it's possible some might exist in your sample but the greatest error with these kits is the human factor (that's also been shown over & over
) ... FWIW my kit test results pretty much agree with my local water report .... & no, they aren't using kit technology ... hundreds of samples are collected & analysed to form the annual report, so it is representative of the water actually coming out of my tap & entering my tank
)
