An Alarmone is an intracellular signal molecule that is produced due to harsh environmental factors. They regulate the gene expression at transcription level. A riboswitch is a regulatory segment of a messenger RNA molecule that binds a small molecule, resulting in a change in production of the proteins encoded by the mRNA, it works as a way to sense eviromental changes.
I've been reading some papers ("ZTP 5-Amino 4-lmidazole Carboxamide Riboside S-Triphosphate): A Proposed Alarmone for 1O-Formyl-Tetrahydrofolate Deficiency" , Bochner 1982; "An Ancient Riboswitch Class in Bacteria Regulates Purine Biosynthesis and One-Carbon Metabolism" Kim 2015) which study how ZTP can bring changes upon 10f-THF deficiency. Bochner (1982) through several experiments proposes ZTP as an alarmone for 10f-THF; some assumptions of Bochner might not have been entirely correct and a later paper fails to replicate some experiments. Later Kim (2015) propose ZTP as a riboswitch.
I'm not fully aware of the differences between the concept of "alarmone" and "riboswitch". Seems to me that both Bochner and Kim propose virtually the same thesis.