The artificial plant in my tropical tank has gone black!!??!?

will this hurt my fish? what is it? i keep a betta and a panda corydora.when i last did a water chang i noticed a few black dots on the plant but i just thought it was poo! now 8 days on its covered! im doing another change tonight, should i put the plant back in after it cleant?the plant is an azoo rubber anemone. also for the past few days my cory has been chewing my betta's fins? they have been together over a month and this hasnt happened before! do you think he isnt getting enough food? i feed my betta floating pellets and then add some tetra flakes after, the betta wont touch the flakes so he is not being over fed! please help!