I do agree that people should dress appropriately for Shul this includes all manner of dress not just what is on their feet while standing in front of the King (with a capital “K”) of all kings (with a small “k”).
However, be happy people are coming to Shul. Perhaps they all have a valid reason for wearing crocs.
Now, if this still bothers you so much to the point that you forgot in front of Whom you are standing – then I suggest that you do not attend Shul M’Shabbos – not this one coming up but the next one – the 9th of Menachem Av. Additionally, you may want to avoid attending on the 10th of Tishrei also.
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and a reminder to all, remember to bring your non-leather footwear (slippers, canvas sneakers, dare I say it – crocs, etc.) to Shul with you E’Shabbos on the 7th of Menachem-Av, as you will need them once Shabbos is over.