HELP WITH ANOTHER CREATURE

[RICKY81]

i have had some pink lemonade paly's for about 6 months already and they were doing really good until about a week ago that they started to die off. A few minutes ago when my lights turned off i went over to the tank and was watching (moonlights on) when i noticed some very small creatures on the pink lemonades one that measured about 1/4 inch and another one about 1/8 inch, they are clear with four legs and 2 arms (something like roaches but clear and curved) and as soon as i turned on the lights (T5's) they ran very very fast behind the rocks. then i turned the lights off and they come out turned them on and they hide.... what could this things be? are they good/or bad? what can i do about them. tried taking a picture but couldn't get them.....thanks for any information provided. tank has been running for about 1.4yrs ( i have other zoas and palys and they are doing good)