Nice gameplay loop. Initially I didn't know if my upgrades are working but then I realized they happen every now and then and are not permanent.
I did find it a bit hard, especially the rate at which they grow is too high. The growth rate made me go for random clicks in the area instead of aiming certain amoebas, which was not efficient as sometimes my upgrades worked and removed amoeba in that area. So it felt a bit random at times.
Edit: One thing that might help is highlighting the cells that will be destroyed in the next click. So if the row is gonna be destroyed, the whole row is highlighted etc. Takes out the guessing game out.