It's overpotted and watered too much. Monitor it for spreading - if it does, you'll have to cut out the rot and reroot. BTW, you might want to be patient with replies - no one here is paid to answer questions, so sometimes a response may take a while.

Hey guys! I recently bought this adorable little cactus, and am trying to figure out how to care for it. Ever since I placed it on the windowstill, it started growing a bunch of little cacti around it so it seems to like strong heat and sunlight. I have to repot it soon because of the growing number of cacti, and I'm trying to identify it so that I know how to best care for it :) If anyone knows what this little thing is and have any care tips for it, I'd really appreciate it!

Looks like a Trichocereus of some kind. Doesn't quite look like the common, but still beautiful T. grandiflores. Your pot looks good along with the size. Give as much light as you possibly can. Soil should be fast draining. A good soil mix would be 50% perlite or pumice and 50% C&S soil. I don't know where you live so I dont really want to tell you when to water. I live in an area where I get zero rainfall from May-October and during these months I water my Thrichocereus once every 10 days. Your plant looks nice and healthy.your cactus looks juvenile still so don't expect blooms this year, but blooms will come eventually. Goodluck:).

Yes, that's what I meant. Sometimes, ppl have only 1 or very few plants and do not want to buy whole bag of new soil. But I can see that you re-potted, so I am assuming you bought that soil mix. There are many bagged soils for sale. It is difficult to decide what to buy - and many do not work as promised. This one is supposed to absorb more water - not so great for succulents. But, if you already have it, and do not have other use for it, make sure you add perlite as advised, maybe even more than 50%. You could also add some grit - not sand - so there will be more large particles in the mix. When thinking of watering, check deep inside of pot (at least 3/4 deep) with a bamboo skewer to see if it has dried up. If it comes up wet/cold and may have some particles sticking to it, there is still plenty of moisture.