It could be a fungus, it’s tough to tell but it looks similar to fungus that I’ve encountered. I use this product and it works well! https://www.amazon.com/Advanced-Disease-Control-Concentrate-32-Ounce/dp/B000NCUW6M/ref=mp_s_a_1_3?dchild=1&keywords=bioadvanced+fungicide&qid=1610031151&sprefix=bioadvanced+fun&sr=8-3
Yup. Definitely fusarium. If you live in a place where you can purchase antifungals, buy a systemic and soak your plant in that. I have had success with this one, which is also available at Lowe's in the US. Fusarium is a pain, but not the end of the world. If you treat it, your plant can usually make a comeback.
Discarding the plant is fine, too, of course! Use bleach or peroxide to kill any fungal spores on your equipment.
Those dark brown spots look like fungus to me. I’m battling fungus on many of my succs right now and those spots look identical to it ): I started using this product but it hasn’t been long enough for me to speak to its effectiveness. https://www.amazon.com/Advanced-Disease-Control-Concentrate-32-Ounce/dp/B000NCUW6M/ref=sxts_sxwds-bia-wc-p13n1_0?cv_ct_cx=fungicide&dchild=1&keywords=fungicide&pd_rd_i=B000NCUW6M&pd_rd_r=06e8c063-5f08-42be-9398-4c798f5aa80f&pd_...
If you have other succs I would give them a look and see if any others have it.
I water it once a week, pour in 300ml of water that I let sit for at least 24 hours beforehand.
I used this product: https://www.amazon.com/gp/product/B000NCUW6M/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
I followed the label, sprayed it once every 2 weeks, did 3 sprays total. Nothing happened.