Well, you haven't said how much light for how long it gets daily, what kind of soil mix it's in, or how often and HOW you water it. Without any info., it's hard to advise.

hey there, sorry i don't have much of an answer for you. but was just wondering where you were able to obtain a starter mimosa? i had started one of these from seed when i lived in virginia, but unfortunately had to leave it behind when i moved to the midwest during the winter. but would be interested in obtaining one for possible use as a bonsai.

I would think that first off it need not be in the house. A Mimosa is a deciduous tree, the ones here are not growing like crazy right now. Has this tree been subject to an artificial spring in your house?
In the spring, I would take the tree outside and next winter find another method of over-wintering the tree. I do not think it will survive as a houseplant.

i check every day and i water it as soon as it is dry. it is next to a large window in a warm room that faces south. it is in the soil i got it in. (it is still small) i use regular house plant food. i actually read that these were evergreen tropicals. so ikept it indoors. it was forced in to an un-natural spring. i guess it is a relief to hear that these are deciduous trees, so im not as worried. i will move it out side as soon as it gets warm. thank you...
also i got it here from a garden center here in Kansas City MO. although i dont know if they really make all that great of bonsai when grown from cutting.

Is it possible you have a 'sensitive' plant that folds its leaves when you touch it? That IS a Mimosa, but in fact just a house plant. If you have an Acacia dealbata, which is very different, and is used for bonsai, then you can bonsai it (it just looks like mimosa and is often called that).