They're the same caps except that the green one is a better material so it shouldn't crack as the grey ones typically do in time. Replacing that cap can't cause a problem but you're probably seeing the low coolant light because you either need to add a bit more coolant (just past seem, halfway) or there's a problem with the bottles level sensor, not a big deal.
Don't overtighten that cap as there's no need to do that. If the seal looks good, just tighten it normally, snug.
If you have another coolant leak elsewhere, you should have the system pressure tested if the leak source isn't obvious but perhaps just topping it up now will suffice? Keep an eye on its level once topped up and see.