Is it permissible to daven with crocs?
When we daven we have to dress in a respectable manner. Therefore we have to dress in a way that we would greet a respectable person. Therefore in countries that a person would not greet an important person with crocs, then it is also improper (although not prohibited) to daven that way.
M”B 91-12; Kerem Shlomo comments on O.C. 91-5 brought in Eishei Yisroel 10 footnote 22.