When you say they are multiplying like crazy what do you mean? 1 a month or 3 a day? It is hard to tell from the pic but a quick google search shows that they kinda look like manjo anemones. Check them out and see if it matches up.
If mushrooms multiply that fast in other peoples tanks I must really be doing something wrong. Either that or they are manjos. Hopefully someone who has had them before will chime in soon and give you a positive or negative on the Manjo thing.
Joes juice or aiptasia X will take care of them pretty quick if they are manjos, but make sure to get ahead of the problem. I have had more luck with the Aiptasia X simply because of the applicator. The Joes juice applicator seems to clog up just when you need it to work the most.
I dont think they are manjos. They look like some type of purple ricordea mushroom to me. They will move and let pieces of their foot behind so thats probably how you are seeing so many babies popping up.