R' Tzvi Pesach Frank (Har Tz'vi OC §106) would seem to hold that you should make another ha'eitz. The reason is that you intended that a different b'racha would be needed to cover the mango and that your original ha'eitz wouldn't cover it.1
R' Ben Tzion Abba Shaul (Or L'Tziyon 14:15) is less certain (he brings arguments each way), and he concludes that one should simply avoid eating the mango. If possible, he adds, one could find some ha'adama or shehakol food on which to make a more generic blessing, having in mind to also exempt the mango just in case.1,2
1 The cases discussed there involve (1) a person who recited a ha'adama on a fruit, thinking that particular type of fruit required a ha'adama blessing, and (2) a person who recited a ha'adama on a fruit by accident when he meant to recite ha'eitz. These responsa address how to proceed in both cases with respect to eating other ha'eitz and ha'adama foods. Those cases are slightly different from this case, but the reasoning discussed there seems to apply here.
2 Alternatively, one might make a second ha'eitz on some other type of fruit he did not previously plan on eating at all (e.g., a kiwi), and this would exempt the mango. However, while this might be a preferable method in one sense (because it uses the more appropriate specific blessing of ha'eitz), it may also pose a halachic question: The halacha likely follows the opinions that the original ha'eitz covers all types of fruits that would be brought to the table while the fruit salad is still not fully consumed (or possibly even afterwards), except for those types of fruits the person expressly or implicitly did not intend to be covered by the original ha'eitz blessing (such as the mango). If so, then reciting another ha'eitz on some other type of fruit (such as the kiwi) would involve an unnecessary, forbidden blessing (see Mishna B'rura 206:22, Sha'ar HaTziyun ad loc. 23-25). Therefore, it appears best to use a ha'adama or shehakol food for this "just-in-case" blessing to exempt the mango.