In the nominative after an indefinite article, they just differ in gender ("-e" feminine, "-er" masculine). But there are other possibilities too, have a look at the declination tables: https://de.wiktionary.org/wiki/Alleinstehende https://de.wiktionary.org/wiki/Alleinstehender

Hi Jian, Both come from the same adjective (alleinstehend). In nominative case Alleinstehender is the masculine declension of the adjective and alleinstehende can be either the feminine one or the masculine if before the adjective comes the article that establishes the gender of the subject. You can look for an "adjektivdeklination tabelle" to find some examples.