did you light acclimate them?, where they shipped to you or store bought? if they where shipped to you then they might be stressed and need time to recover, check water quality, altough it might not be a param thats out of whack but just to be sure, if everything is a ok then i would consider letting them be and be patient for them to get acostumed to your water, flow and light
FWIW the body on my green slimer is tan color too, only the polyps are the ones that are green and that is what gives it it's color, sorry i dont have a bonsai
good luck
sana