% !TeX TS-program = lualatex
\documentclass{beamer}
\usepackage{xcolor}
\usepackage{luacolor,lua-ul}

\setbeamercolor{alerted text}{fg=yellow,bg=cyan}

\renewcommand<>{\alert}[1]{{%
  \usebeamercolor{alerted text}%
  \LuaULSetHighLightColor{alerted text.bg}%
  \alt#2{\highLight{\color{alerted text.fg}#1}}{#1}%
}}



\begin{document}

\begin{frame}
\alert{highlighted text}

\alert<2>{Some text} highlighted in the second overlay.
\end{frame}

\end{document}