I have riccia growing in a tank that receives only indirect sunlight. It grows much looser and doesn't form the tight mats that it does when under good lighting.
You can tie riccia down to something but it tends to come loose and spread around your tank when pieces of it break off.
Some people hate it. I am not one of them. I used to let it grow in thick mats at the surface of my sorority and my females used to slither across it to get to the front of the tank.
Is your moss ball marimo or actual moss? If it is moss, it could be that there is a polystyrene ball or something they have attached the moss to and this is why it won't sink. I have seen that done before.
sometimes holding it underwater and squeezing it will make it sink. If not it should sink in time if it isn't the kind mentioned above. For the normal kind squeezing them out of water adds air pockets and makes them float.
Keep yourself clean and bright, for you are the window through which you must see the world.
I tied my riccia to round rock with fishing line, to a driftwood branch with fishing line, and clamped between a mesh thing i got at LFS with 1/2 inch holes and they are growing great, only a few pieces drift loose now and then and I just scoop them out and throw them in a bowl. You can also get stainless steel mesh then sandwich the riccia between that and a plastic mesh like the stuff those shower washy things are made of for a carpet.
My light is an LED 7000k 9 watts which made the riccia grow like crazy, I attached a picture of the growth between 3/7 and 4/1
fluval moss ball - it's not a real moss bal, its just a carbon filter. if you just let it soak in the tank for a while it should eventually sink. (and don't run it hot water ever I destroyed the shape of mine that way :/ )