I think Aglaonema. Does your pot have a drain hole? The moss/fungus on the surface leads me to believe this plant is staying way too soggy way too often in a soil of mostly fine particles, no air pockets in the soil. This plant could use completely new soil as soon as possible. Until then, allow it to dry much more than you have been.
Is it possible that it got sunburned? If it was suddenly put into much more light, that could happen although, once acclimated, this plant can handle some direct sun.
Aglaonema, most probably A. simplex. The two genera Dieffenbachia and Aglaonema are often confused by easily separated by latter having few (usually less than 10 pairs) lateral veins and former with more than 10 often up to 20 pairs of lateral veins
Probably yes, A. modestum, although two look similar in leaves and are differentiated easily only in flower (spadix shorter than spathe, male part 3-4 mm thick in A. modestum ; longer than spathe and male part 9-10 mm thick in A. simplex), and it being most widely cultivated.
This is not a big plant and none of the leaves are very large (shorter than my finger). Also, the leaves are quite determined to grow downward; they are not droopy at all - if I try to push them upward, they offer resistance.
I agree with Dieffenbachia but do not know for sure.
I also agree with overwatering or poorly draining soil. It is very easy to overwater plants, make sure you have a well draining soil and I would mist daily and water about once per week.
I wonder the great American authority on cultivated plants, and founder of Bailey Hortorum which has published Hortus Third won't be knowing difference between Diefenbachia and Aglaonema when he (L. H. Bailey) published 'Manual of Cultivated Plants'. I can count not more than 5 pairs of lateral veins. It is Aglaonema modestum and not Dieffenbachia on any counts.
I am uploading both common species of Dieffenbachia
First D. amoena
Second D. maculata
Those still sticking to Dieffenbachia may kindly count the number of veins (f they have faith in L. H. Bailey) and take considered decision
With all due respect to you, and certainly with respect to the dean of American horticulture, L. H. Bailey, that work is sadly out-of-date. Aroid botany is a fast evolving field, and is based, for the most part, on flower morphology. Genetic testing methods have further moved the science of taxonomy away from simple morphology. Bailey did not have access to the full number of species of either genus. In his day, his key for the two genera may have been valid. Is is not now.
Additionally, new horticultural varieties may have mutations that significantly alter the characteristics of the wild-type plants. Bailey never saw the modern Thai hybrid Aglaonemas, or the micro-miniature Dieffenbachias.
The plant in the picture is very weak and light-deprived; it may not be expressing its normal appearance. It may, when stronger, have more veins. It does show a type of variegation that is charachteristic of Dieffenbachias.
Bailey may be outdated for some (although with University teaching experience of more than 35 years and practicing taxonomist for last 42 years, I know basics don't change, inspite of development of numerous cultivars) but I Think the Flora of China published in January, 2011 is not outdated. Pl. see page 23 of this
Aglaonema modestum "...; leaf blade pale green abaxially, green adaxially, ovate to ovate-lanceolate, base obtuse or broadly cuneate, apex acuminate; primary lateral veins 4 or 5 per side, ascending and arching, ..."
Angelfish, this debate over the ID is interesting but not necessary to care for your plant well.
If you're not sure if the pot has a drain hole, I'm sure it doesn't, otherwise you would have seen water spilling out of the bottom onto the table. The pot is beautiful but plants are not happy, long-term, in pots without holes in the bottom. Changing the soil fairly often is necessary (and making sure not to add too much water at any one time) in lieu of a drain hole, and very tricky for anyone to attempt. I stand by my original advice to water less, less often, and change the soil. And with others' advice of more light although that needs to be done somewhat gradually or sunburn can occur.
Would you be able to add a pic showing the pattern on the leaves, like looking down from the top?