"alarming stage" is correct and usable in written English.
You can use it to describe a point in a situation which is particularly concerning. For example, "The increasing spread of the pandemic to developing countries is an alarming stage in this crisis."
In Indonesia, the rate of accident and serious incident of aircraft have reached alarming stage.
The lack of goals had reached an alarming stage for the Devils, who had failed to score more than two in seven straight games.
As a consequence, the research on quality and quantity of groundwater is in alarming stage.
Resistance to available antibiotics is increasing at a very alarming stage globally (Stuart and Bonnie 2004).
Resistance to available antibiotics is increasing at a very alarming stage globally [ 1].
Bacterial resistance is now at the alarming stage due to the irrational use of antibiotics.
In the 1960s, a number of soil profiles from all over the Scheme, contained more salts in the subsurface than measured during the initial soil survey, indicating a disturbing tendency, although not alarming at that stage.
In the ensuing years, Kim had proven one of the most enigmatic figures on the world stage, alarming world leaders with his focus on militarization and nuclear armament.
The most alarming thing at this stage of the game is Zanu-PF's incredible persistence in stage-managing what has obviously been a rout.
The alarming problem of end stage renal disease in Hispanics is dramatically increasing because they are more frequently affected by diabetes and high blood pressure, the leading causes of chronic kidney disease.
This view frames the menopause as just another biological stage, no more alarming than any other.
