Ohiorn, hey those are polyp eating nudibranchs!! I know b/c I have them too, the exact same ones, great pic of it. They have been eating and killing two of my zoo colonies. So far I have been picking them out with tweezers, which doesn't get them fast enough, and if you have two then you have eggs as well. I did an Iodine dip for 7 minutes on my to colonies with good results! Still pulling one or two out with tweezers, had to have been eggs b/c I have killed about 25-30 of em. My guys at the LFS said to do the iodine and tweezers and if all else fails a freshwater dip. Said it will take a month or two to eradicate them all. HTH. Any other questions let me know.