Firstly, I have not a deep math knowledge, so take my words with one or two grains of salt
1) you should provide use with some more info about your data;
2) I am wondering why you used spearman and then switched to another kind of measure of association
3) do you data meet the assumptions of linear correlation?
4) what value of Spearman's r did you get from your data?
Whit these questions in mind, may be that your two varialbles are monotonically related (this would explain the significant rs; but I would like to know what is the value of rs) but the relationship is not linear (this would explain the R2 value). See this page for further info about Spearman vs Pearson correlation:
Hoping that this can help you a bit